I'm not sure if this applies to all Preludes, but it might be of some use:
In order to get the code you'll need to find the service connector. On the 5th Gens it's on the right hand side of the centre console underneath a carpeted panel that just pops right out. The connector looks like a telephone connector and is plugged into a blue plastic piece that just serves as a holder. Unplug it and you'll see that it has two sockets in it that lead to two wires.
http://www.hondaprelude.to/main_checkengine.html