I used Cubase VST/24 v4.1r2 which was the version before VST 32 5.0 and you could not import mp3's directly and my guess is that VST 32 5.0 is the same.
I'd check the index of the manual for mp3 to be sure.
What you can do is use a format convertor program to convert the mp3 to a WAV or AIF and you will be able to import that. There are probably some convertors at http://www.hitsquad.com/