Hi Dwalbert,
You don't say whether anything else is disabled or whether the Merge & Centre button is only disabled on a specific worksheet or for every worksheet (even new ones).
If it's on a specific worksheet that you are using, the simplest reason could be that the sheet is protected. (To see if it is, on the menu select "Tools" then "Protection". If you are given the option to "Protect Sheet", then the sheet isn't currently protected...but if you are given the option to "Unprotect Sheet", then the sheet is currently protected.
Anyway, that's probably the first thing to check out here.
Regards,
Blaine