Ok, here's what I did to get "back to my mac" to work correctly. Initially, I was getting a Double NAT error. So I bypassed my Time Capsule altogether and plugged my ethernet cable directly into my iMac from my AT&T DSL modem. After having done this, I was still getting a NAT error, although it wasn't a double NAT this time. So I went into my modem and changed only a single setting. I changed(on att's user configuration page) from a private address to a public one(it was set to private by default). After I made this change, I was able to get a green bubble next to "back to my mac". So, so far I had eliminated the router... I was getting a green bubble and "back to my mac" was working correctly, but I still had to hook up my Time Capsule. I then removed the ethernet cable from my iMac and inserted it back into my Time Capsule. I immediately got a red bubble and an error stating that I wasn't connected to the internet. But I wasn't sweating it, because I had narrowed the issue to my Time Capsule and all I had to do was set it right for this to work. So under the "internet connection tab" in the airport utility, I set my connection to PPPoe rather than the default "ethernet" setting. I then entered my att account's email address and password that I made when I initially set up my DSL service. Under the PPPoE tab, I entered the DNS Servers, which I got from att's configuration page. I checked "Enable default host at" under the NAT tab, added my mobileme username(including the @me.com) and password... clicked update, waited for 30 seconds and I had a green bubble next to "back to my mac" after my Time Capsule rebooted.
One last thing. Go to finder preferences and under "SHARED", uncheck connected servers and bonjour computers. Leave "Back to My Mac" checked. Doing this will only show your connected computers and drives when "back to my mac" is actually working correctly. I initially thought it was working because I saw my computers in the side bar, but they were there because I had connected servers and bonjour computers checked and not because "back to my mac" was actually working correctly.
I sure hope this helps someone.