In order for Toast to burn a VCD, the movie must be in a valid "mpeg-1" format. As you have Toast, you should also have the Toast VCD QuickTime plugin which will permit you to export from iMovie and QT Player (Pro) in the appropriate format.
Note that some movies may already be in some sort of multiplexed format (like mpeg-2) that can't be converted by exporting from QT Player Pro as the separate "streams" of A/V cannot be "transcoded" in this manner.
and link to the various pages and external sites where the appropriate software (all of it freeware/shareware and running in OSX) can be used to de- ,trans-, and encode what you have into what you need for VCD and SVCD.