Ok... since I finally figured out the OMP friend challenge thingie, I though it appropriate to post a "How-To" about it. Do things right and you'll see the pic directly below - in this case, my alter-ego Jeffsiamese (MRR-63) challenging the real Jeffsiamese. Some pics are from the other account 'cause I snapped them faster, but they're all in order.

Your friend should know when you're sending the invite or vice-versa... seems the only way to make it work. Here's the first screen:

Go ahead and send your invite:

Be quick! That invite comes and goes in no more that three or four seconds, then it's gone! Accept very very quickly!

You'll then see this screen:

And me taking my time picking which car I'm gonna whip butt with!

J.

P.S. The screen showing that only series A is unlocked is the Jeffsiamese (MRR 63) one - I started it from scratch, sent invites back and forth, and it's the only one I was quick enough to snap a pic of. A bit confusing, but you get the gist of everything - not really complicated at all. 

J.