Your MMSI number is entered into your radio. Be sure to read the
instructions and get it right. It will only take three entries, after
that it'll lock up and won't accept a number. This is to keep people
from changing them. You don't enter any number into an EPIRB, it comes
with it's own ID, and you register it to you.
Be sure to interface your GPS with your VHF, so if you hit the
emergency button, it'll transmit your position as well.