If you are running 10.1 then you will need SP1 and then download this patch:
ArcGIS 10.1 SP1 (Desktop, Engine, Reader) Set Bing Key Patch
http://support.esri.com/en/downloads/patches-servicepacks/view/productid/160/metaid/2003
You should then find an executable called SetBingKey.exe in the \bin folder in the location you installed ArcGIS.
The default location is Program Files (x86)\ArcGIS\Desktop10.1\bin
The resource that @ciava.at provided has all this information for different ArcGIS releases.