As I mentioned above go here:
http://blog.tweefari.com/post/3626600209/t-mobile-usa-carrier-bundle
At the bottom you will see bundles. Choose the iOS version you currently have. From their readme:
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How to Enable Carrier Bundle Support in iTunes for Mac OS X
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Step 1: Quit iTunes.
Step 2: Launch Terminal.app found in /Applications/Utilities folder.
Step 3: Enter the following command into the Terminal window:
defaults write com.apple.iTunes carrier-testing -bool TRUE
Step 4: Connect the iPhone and launch iTunes.
Step 5: Select the iPhone from the list of devices on the left then hold down the Option key and click the "Check for Update" button.
Step 6: Select the proper .ipcc file.
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How to Enable Carrier Bundle Support in iTunes for Windows
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Step 1: Quit iTunes.
Step 2: Press Windows Key + R to open the Run dialog.
"C:\Program Files\iTunes\iTunes.exe" /setPrefInt carrier-testing 1
OR
"C:\Program Files (x86)\iTunes\iTunes.exe" /setPrefInt carrier-testing 1
Step 3: Connect the iPhone and click to launch iTunes from your Start Menu.
Step 4: Select the iPhone from the list of devices on the left then hold down the Shift key and click the "Check for Update" button.
Step 5: Select the proper .ipcc file.
I used this one on my sisters 3G and it works fine:
t-mobile_us_standard.ipcc