Which version of PrE are you using, and what is your OS (Operating System)?
Next, what is your source footage, and your PrE Project's Settings?
There are several possible MPEG settings, and how you plan on delivering your finished file, will often dictate which you choose.
In the Share (Publish + Share in PrE 11) panel, under Computer, you should have most of the MPEG settings, but might have to scroll down in that panel, to see those.
I'm with Bill. How you plan to share or deliver your video determines which output setting is best.
For instance, if you are planning to share your video online using a site like YouTube, Vimeo or Facebook, an MPEG is not a good output format at all! (This is related to interlacing.)