time capsule NAT Port Mapping Protocol (NAT-PMP)

Does setting up back to my mac on an airport time capsule mean the same thing at NAT Port Mapping Protocol (NAT-PMP)?

I'm trying to do screen sharing with my father-in-law's computer. We both have iMacs and are both using a new airport time capsule as our wireless router.

I could get his time capsule to show up on my finder window, but his iMac never did.

Any thoughts on what I am doing wrong?

Thanks,

Paul