Opening port with Mac Pro as router!

I have two routers, both Apple - and neither work with my Comcast modem here in Colorado Springs.

So as a working awkward solution, I set the modem up to run into my Mac Pro directly and then use the Sharing preference panel to broadcast the signal out to my MBP and iOS devices.

I'm trying to set up Air Video again (an iOS app to watch videos from home while out and about,) but I need to open the port. So the Mac Pro is my router, how do I open up a port? I can't figure out where..

thanks