What spaces are you referring to? There aren't any in your example!
Unless you're referring to the ones on either side of "/ADD"?
For starters, you've missed out the backslash in your example:
If you use the "Start -> Run" command, the following command line example should work perfectly ok
(it does for me & many many others)
"C:\Program Files\Winamp\winamp.exe" /ADD "C:\My Music\MP3\track.mp3"
Quotation marks are necessary for paths with spaces.
Otherwise, you can use (eg):
C:\Progra~1\Winamp\winamp.exe /ADD C:\MP3\track.mp3
All available command line parameters are explained here: