The WRT54G has two serial ports you can access via the 10 pin JP1 header which is found near the front of the router board next to the lights. I ordered some 2×5 10 way headers from ebay.

Here is a picture of where JP2 is, it's the 10 way one on the right the one on the left is a 12 way port used for JTAG which i'll cover later.

The holes in mine weren't pre soldered but i've seen others that have been so if they are you'll need to remove it first. It's then a case of inserting the header with the shortest pins exiting the bottom of the board and then soldering each one.

and how it looks from the top