I know this is an old thread, but I just had the same problem with ITC Avante Garde Bold, and have just now solved it in Motion 4, so thought I’d better share it just in case others are pulling their hair out like I was!
I downloaded the font in Open Type format (.otf), unzipped the file, then installed it using Fontbook. I then converted it online to a True Type format (.ttf). Here’s where the problem is – you’ve got two versions of the same font on the computer, and when I typed the word ‘COMMUNICATION’ in Motion, the ends of the ‘C’s were cut off!
To solve it, open Fontbook, find the font, right click and select ‘Validate Font’, as Mark suggests above. Check the box next to the .otf version, and click ‘Remove Checked’ at the bottom of that window. Then open up Motion again, and your font should work. At least mine did!