I was able to configure mine just fine today, but I'll admit it took me a few tries in the past before I figured it out.
Go to Start --> Settings --> Connections tab --> Connections.
Since you're not using the standard T-Mobile ROM, just adapt the steps a bit to match up with what is on the WM6 ROM. Where it says T-Mobile GRPS in the instructions, actually use the My Work Network section on the device. You can also rename My Work Section to T-Mobile GRPS if you want.
Under that section, you should see:
Add a new modem connection
Add a new VPN server connection
Edit my proxy server
Other than the My Work Network/T-Mobile GPRS thing, the steps were exactly the same.
Oh yeah, instead of 216.165.155.050 enter 18.104.22.168.
I did that and I was able to get an Edge connection and send/receive MMS messages from my Verizon phone.