I received an email to Ask Dave inquiring about the key sequence to unlock the keyboard. The radios come from the manufacturer with the keyboard locked so that it complies with FCC rules for those outside ham radio who are not allowed to program their own radios. Hams, however, are permitted to program their own radios.
The instructions are found on Page 38 in the manual. You press the Menu and Star keys together. Here is the relevant extract from the manual.
As far as I can tell (and I could easily be wrong about this), once the programming is unlocked the radio stays unlocked.