.mpg is a movie file, so I suspect you ended up with spam or worse.

Many sites offer a demo version: test with that rather than some unknown source.

According to RuneSoft Dream Pinball Mac was supposed to be available last summer in a UB through TopWare Interactive Inc. - www.topware.com but doesn't look like the Mac version is listed. The PC version has a demo, so maybe send them an email asking about the availability of a Mac demo.