Unfortunately subtitles created within MC, using the SubCap Generator, cannot be easily imported into AvidDVD.
If you want to work with subtitles in one or more languages and give the viewer the possibility of choosing which they want to have on-screen, AvidDVD offers a better solution.
These 3 tutorials will hopefully give you a little insight into how AvidDVD deals with subtitles.

Part 2. Entering Japanese subtitles on a non-Japanese OS, and Copying & Pasting from other sources.

Regards,
Douglas