Jay, you should know more about this than most- this network access code is a UK/ Euro bit, after all. They never did implement this in north America with the PSP or the PS2.
The network access code should be on a smal red bit of paper included with the PSP. If you can't find it give Sony's tech support a call (number is in the book that came with it) and they should be able to get you a new code.