Hi Toni
I'm no expert on fonts, but Myriad-Roman and Myriad MM will be different fonts in Framemaker's eyes. First, I'd check that you don't actually have Myriad-Roman somewhere on your system. I think it comes with Illustrator or Photoshop, so I'd look in my C:\Program Files\Adobe folder.
If you don't have Myriad-Roman, you can tell FM to substitute Myriad MM instead and see what it looks like. To do this, edit the maker.ini in C:\Program Files\Adobe\FrameMaker. Back the file up first!
You can change the default font for substitution by editing:
DefaultFamily=Times New Roman, Tms Rmn
Or I think you can fool FM into thinking that Myriad MM is Myriad-Roman by putting an entry in the section called (sorry I can't give syntax details, I'd have to experiment):
[WindowsToFrameFontAliases]
Hope this helps!
Rebecca