For what it's worth, the Inventor format listed is the 2016 version of Inventor, which isn't backwards compatible with 2015. 2015 is what we have on our school computers.
The STEP import is working fine for me, and even comes through with proper color. So I'll work to upload the 2015 Inventor version of the field to my GrabCAD tonight, then publish it so it's available at our team CAD library link in my signature.
Edit - turns out the upload was pretty quick -
Direct Link