The MSL number is used on some carriers to keep you from getting to your phone’s configuration settings.
If you are using an Android and need your MSL number to access your phone’s configuration menu, you can load up a terminal emulator (piles available in the Market) and run:
getprop ril.MSL
BAM! It should spit back your 6 digit MSL number.
Alternatively you can call up your carrier and beg for it. :)