On a Mac just start Terminal (Applications > Utilities) and type
defaults write com.apple.iTunes carrier-testing -bool TRUE
Restart iTunes if you had it open
In Windows:
Press Windows Key + R to open the run dialog. Enter in cmd then click OK
Step 3: Enter the following command into the Command Prompt window.
"C:\Program Files\iTunes\iTunes.exe" /setPrefInt carrier-testing 1
Restart iTunes if open.