MPEG1 is an old format and is not supported by Aperture. You will have to convert them to something that Aperture can handle like .mov.
I believe Quicktime 7 (if you have that version) is capable of doing the convert. If you don;t have QT 7 try opening one of your MPEG1 files in the QuickTime on your machine and see what happens.
If that doesn't work you will need to look around for an application to convert the files.