After much stumbling around, I finally figured out why installing mru.vim into Pathogen wasn't working for me. It was this small comment that finally got me to realize the problem:
Yes, the root of each set of runtime files must sit in
bundle.
I had created a directory in my bundle directory called mru, and inside of that
I put mru.vim. The problem is, since this is a plugin, it needs to go into a
plugin folder inside of there.