The most likely situation is that the .mpp file was created with Microsoft Project 2007. OmniPlan doesn't really support that yet, but if you re-save (or maybe it's an export option) from Project 2007 into the 2003 format, it should work.
If that doesn't help, would it be possible to send a copy of the .mpp file to
omniplan@omnigroup.com so we can figure out what's going wrong?