Help: network setup for specific app

Hello all. Finally got my first iPad (3rd generation). It's splendid, and now I wanna try configuring my network, so I can access my files away from my home network via the Plex iOS app.

I've been following this:

http://wiki.plexapp.com/index.php/ConfigurePortForward

I just can't seem to get it to work. While it's a nice guide, there really are SO many variables in configuring a network, and it's really something I'm not an expert at.

The deal:
Mac Pro 2008 running 10.7.3
Airport Extreme with both Airport Utility 5.6 and 6.0. I use 5.6 because most portforwarding guides I've found use this
60/60 mbit connection
static IP from my ISP (so I've skipped step one)

I'm unsure whether the IPs delivered by the Airport Extreme are static, so I've done step 2 of the guide.

Normally, the router is in bridge mode, but (I guess) if want to be able to configure NAT/DCHP settings I've had to select "share a public IP address" under connection sharing instead of bridge mode. The third option is "distribute a range of IP addresses." Once off bridge mode the lamp on the Airport Extreme is yellow instead of green, but the internet is still working. Airport Utility states its status as "double NAT" hence the yellow light. More on this: "This Apple Wi-Fi base station has a private IP address on its Ethernet WAN port. It is connected to a device or network that is using Network Address Translation (NAT) to provide private IP addresses. You should change your Apple Wi-Fi base station from using DHCP and NAT to bridge mode."

I've done step 3 but somehow it's not forwarding said ports, I'm pretty sure. I did the same step for the torrent application, Transmission, because in this program's preferences you can check to see if the used port is open; it is not.

I know this is all very unscientific and a lot of trial and error, but I'm hoping someone who knows this stuff could point me in the right direction.

Generally, it isn't just for Plex iOS. It'd be really nice to have remote access (files, etc.) to my home computer. It's not an issue leaving it turned on while I'm not at home. Eventually, I may be building a genuine NAS server, so this is sort of the prelude to that.

If you need any info (screenshots, whatever) of my current setup in order to help me, let me know.