Once you have the code it is simply a matter of typing it in, it should come back immediatly. You have 3 tries before you have to reset everything (disconnect the battery).

sometimes the code is on a sticker somewere in the the glove box, its a 5 digit number. or in the case of my integra under the ash tray.

here are a couple of threads about lost radio codes:
http://www.all-acura.com/ftopict-674-radio.html+code
http://www.all-acura.com/ftopict-383-radio.html+code
http://www.all-acura.com/ftopict-447-radio.html+code