Well, unfortunately the HTC Connection Setup app does not allow you to manually edit the APN. It only allows you to select a preset one and as I said before, they do not have a Straight Talk profile as of now.
The only editing you can do on a Windows Phone without an app, is just the actual APN, Proxy, Port, Username, and Password. You cannot edit the MMS APN settings sadly enough.
The APN settings for Straight Talk are the following:
MMS APN: att.mvno
MMSC: I'll have to edit when I have permission to post an http link
MMS Proxy: 126.96.36.199:80
Max Size: 600
Leave the rest of the settings blank, this has worked on other smartphones for full data speeds and MMS compatibility.
I have since factory reset my HTC Trophy and tried connecting to Straight Talk and AT&T with data above Edge, this was unsuccessful; a reset will not fix the radio. The only thing I can think of is that when HTC and Verizon pushed the Mango update on the Trophy, it changed the radio since it is suppose to support AT&T's spectrum. I purchased an HTC Titan on eBay and am now using that as my phone; I'm getting full HSPA+ speeds. The only issue with Straight Talk on Windows Phone is the MMS APN settings, I cannot receive picture or video text messages since it cannot be edited.
Hopefully HTC will add a Straight Talk profile, or allow its customers to change the settings Manually within the App.
Good luck to anyone with an unlocked Trophy, I cannot get above edge data or MMS working on Straight Talk.