I know you're talking about the Dimond, but I did run into this problem on my Vouge. To get it to work, I used the program
QuickMenu. Once I had that installed, I clicked Start, QuickMenu, and an option that says "Execute" or something similar. It might not be under QuickMenu, I can't remember exactly and I don't have it installed right now, but it is somewhere under Start. From there, I browsed to
dmrouter.exe in the Windows folder. Selected that, and in the parameter field, typed
/DIAG. Once I had that, I clicked execute, and my phone went into DIAG mode. Then, to get out of DIAG mode, follow the same steps above then exit.