I am attempting to install Blackberry Curve as a modem on Windows Vista.

The Blackberry is connected and installed, the program is active. I have assured that the connection is working. Step eight, however, is where that stops.

8. IMPORTANT SPECIAL CONFIGURATION STEP
Click Properties -> Advanced. Add only ONE of the following Extra initialization commands with the appropriate carrier-specific APN Setting:

TMOBILE USA Users (Success!)
+cgdcont=1,"IP","wap.voicestream.com"
This is a newer string from TMO that works: +cgdcont=1,,"wap.voicestream.com"
Another alternate string (required for BlackBerry 8100 Pearl & BlackBerry 8800): +cgdcont=1,"IP","wap.voicestream.com"

I cannot enter the string because the text-box does not respond; it is grayed out to signify that it will not accept input.

How do I remedy this?

Belay that. Problem is now remedied.