MP3 is an audio format. Not a video format, so I'm a little confused about this one...
After Effects does support MP3 audio, but not in the trial (because the MP3 decoder is licensed from a third party, and thus can only be enabled when you buy the application). So, except for the trial, you wouldn't have to convert it to anything, you can import those files and use them directly - internally, After Effects will conform MP3 audio so that is converted into linear PCM (ie, just like AIFF or WAV uncompressed audio). But you can definitely live without having to know this.
If you're talking about video footage, you have to check what format is it. Because again, MP3 is audio only.
Well, it's .mpg out of AE, but in AE it says mp3 for the format. When I click on it, it says "file may be unsupported"
It may be unsupported, because (if I remember correctly) is a trial which lacks the MPEG decoders (because of the licensing reasons I mentioned in the previous message).
Sorry, to use such material with the trial, you may have to use an utility to convert them to Quicktme or AVI files. Like the free MPEG Streamclip utility.
If that application also reports the file as MP3 audio, then obviously there was a mistake when exporting the file.
Again, this limitation doesn't exist when you purchase the product. You can use most flavors of MPEG video directly.