I used to have XP for quite some time and the enqueue in winamp used to show up in the context menu just fine. Then I moved on to Windows 7 and the enqueue in winamp option disappeared from the context menu even I had setup winamp correctly for it.
After doing some digging, I narrowed down all the possible causes for this behavior to one program and one simple solution to bring the Enqueue in Winamp back to the context menu. And that one program which is messing the enqueue in winamp in the context menu is Windows Media Player. It seems that somehow it uses brute force to take over all enqueue options for audio files to itself. (Or the context menu simply has hardcoded limit how many options it can show in the first section) But whatever the case is the solution is here.
BEHOLD THE SIMPLE SOLUTION:
1. At first you want to make sure your winamp is setup correctly. That means you have "Show Winamp in the folder context menus in Windows Explorer" option selected under Options -> Preferences -> File Types.

2. Then go to the Shell Options and make sure you have Enqueue in Winamp selected for files. In the following picture you can also see that even the options are selected the Enqueue in Winamp isn't showing on the context menu.

3. The you need to follow this simple guide how to disable Windows Media Player aka WMP from showing in the context menu.
And just in case that site for some reason disappears I'll quote it here:
4. If you did everything correctly, you should be done now. As you can see from the following picture the Enqueue in Winamp now shows up correctly in the Window's context menu and we also got rid of that WMP's own unneeded context menu entries.

IF you want to keep the WMP entries in the context menu, I don't know a solution for that. It might require going to registry files and tweaking context menu from there to give room for Winamp's context menu entry or simply removing the WMP's brute force method of grapping all the enqueue options for audio files to itself. Whatever the case is I'm not going to cover that in this solution.
I hope it works for you too as it did for me.
After doing some digging, I narrowed down all the possible causes for this behavior to one program and one simple solution to bring the Enqueue in Winamp back to the context menu. And that one program which is messing the enqueue in winamp in the context menu is Windows Media Player. It seems that somehow it uses brute force to take over all enqueue options for audio files to itself. (Or the context menu simply has hardcoded limit how many options it can show in the first section) But whatever the case is the solution is here.
BEHOLD THE SIMPLE SOLUTION:
1. At first you want to make sure your winamp is setup correctly. That means you have "Show Winamp in the folder context menus in Windows Explorer" option selected under Options -> Preferences -> File Types.

2. Then go to the Shell Options and make sure you have Enqueue in Winamp selected for files. In the following picture you can also see that even the options are selected the Enqueue in Winamp isn't showing on the context menu.

3. The you need to follow this simple guide how to disable Windows Media Player aka WMP from showing in the context menu.
And just in case that site for some reason disappears I'll quote it here:
Remove Windows Media Player from Context Menu in Windows 7
Windows 7 can do much more than many people think. Do you know of the “Default Programs” control panel for example?
1. Click on Start and click on Default Programs (optionally enter default programs into the search field)
2. This where you can set many of the default programs in Windows 7, file associations, autoplay settings and much more.
3. Click on Set program access and computer defaults:
Choose Windows 7 Default Programs
4. Click on the option Custom
5. Scroll down until you find the entry Choose a default media player
6. Uncheck the options Enable access to this program for Windows Media Center and Windows Media Player
Default Windows 7 Media Player
You have now successfully removed the entry Add to Windows Media Player list and also Buy Music online.
Windows 7 can do much more than many people think. Do you know of the “Default Programs” control panel for example?
1. Click on Start and click on Default Programs (optionally enter default programs into the search field)
2. This where you can set many of the default programs in Windows 7, file associations, autoplay settings and much more.
3. Click on Set program access and computer defaults:
Choose Windows 7 Default Programs
4. Click on the option Custom
5. Scroll down until you find the entry Choose a default media player
6. Uncheck the options Enable access to this program for Windows Media Center and Windows Media Player
Default Windows 7 Media Player
You have now successfully removed the entry Add to Windows Media Player list and also Buy Music online.

IF you want to keep the WMP entries in the context menu, I don't know a solution for that. It might require going to registry files and tweaking context menu from there to give room for Winamp's context menu entry or simply removing the WMP's brute force method of grapping all the enqueue options for audio files to itself. Whatever the case is I'm not going to cover that in this solution.
I hope it works for you too as it did for me.
