Registering for MTN Ghana mobile money is something done at all the MTN call centers in Ghana. However you can only apply for a Wallet if you are an active MTN Ghana subscriber and only Ghanaian citizens with a valid ID document(ie Voter's ID card, National Identification Card or passport) may apply for a Mobile Money Wallet.
MTN call centers are located here
http://www.mtn.com.gh/SubPage.aspx?pageid=19
http://www.mtn.com.gh/SubPage.aspx?pageid=206