Since the update to WP7.5, HTC pulled the app from the marketplace. I guess they're updating it or it was causing too many errors. I still have it installed on my HTC HD7 and it fixed all the connection issues(including mms) I had on Simple Mobile. I did the same when I had my Surround and was able to find the settings for Straight Talk.
I know your frustration, and it's the very reason I started this thread over at WP7 suggestion forum.
Enable ADC and MMS configuration within the OS!
Every Windows Phone should have connection configuration options from within the OS!
Please vote and share the link. If we get enough votes, MS will see that this is a major issue that can't be ignored, especially when group messaging depends on mms.