For more specific instructions, you'll need to tell us which Merlin Mail you have.
For Merlin Mail, the type that is an external PC, and for the 007MLM card, log into the system administrator's mailbox, press 9 for programming, enter the programming password, press 4 for mailboxes, press 6 to modify, enter the mailbox number and #, press 2 for name, enter the letters of the name, press #, listen to the name read back, press 9 to accept or 6 to re-enter, record the user's name at the tone, press 1 at the end of the recording, and *# to accept everything.
For the letters of the name, use the button that the letter is on, and then the position of the letter on the button. For instance, to enter the name "kegler", k is on the 5 button, and it is the 2nd letter, so you enter 52. E is on the 3 as the 2nd letter, so enter 32. G is 41, L is 53, E is 32, and R (this one is different!) is 72. OK, the 7 button really goes PRSQ -NOT- PQRS like the button says!
Now if you have Merlin Messaging, log into the System Administrator's mailbox, press 9 for programming, press 2 for mailboxes, enter the mailbox number and #, press 1 for the name, 2 for the directory listing, 1 to change the listing, enter the letters as above, except this time the 7 button is really PQRS, press #, listen to the name read back, and *# to approve.
Good luck!