Menu >> Visualization >> Start/Stop plugin
This will show your vis tab again. Now with the vis tab visible, just right click and remove the hide checkmark.
If your visualizer doesn't show in the skin, theres something wrong (obviously
). Are you using Milkdrop or AVS? Or maybe something else? I know Milkdrop have an option (Settings > Milkdrop > Comman Settings > Integrate in winamp skin).