Please excuse my ignorance...here is my question
I'm on a 1 player cabinet, running advmame(advmenu) and I wonder where (and how) I can set up it to accept "player 2" connected with a standard PC joystick on the serial interface... 
Thanks very much
Not sure where I'm supposed to go with this, as you left it pretty vague and wide open.
First off, standard PC joysticks don't connect on a serial interface. They use a DB15 gameport connection. If you don't have a gameport, you need to add a soundcard for around $10. If your gameport is used for your current player 1 controls, I would recommend getting an I-PAC for $40 and re-wiring.
Assuming you have a gameport, you can get a 6-foot joystick extension cable (check
www.pricewatch.com or locally, if there's a
www.microcenter.com near you) and route it to the front of your cab (side of your cab) for a place to plug the joystick in.
The MAME default is to have the joystick control Player 1 so you need to remap it to Player 2 and away from Player 1. For MAME 0.60 and up, this is easiest using controller.ini files, see
http://www.fraggersxtreme.com/arcadepanels/faq.htm, about half way down the page for info on setting these up.
That should get you there! Having said that, this is NOT the way I would go. Consider the following.
Your cab probably already uses an I-PAC or some kind of keyboard encoder with unused inputs for Player 2 available.
From here, you can route half of a cut DB15 (joystick), Mini DB15 (VGA monitor) or DB25 (RS-232, cheaper) extension cable to a port on the panel, similar to what I recommend for the joystick connection.
The other half goes to REAL arcade controls. You can get a Happ joystick and 6 buttons from
www.therealbobroberts.com for under $20 or you can get an off-brand joystick and 10 black buttons from
http://xgaming.net/Merchant2/merchant.mv?Screen=CTGY&Store_Code=XOS&Category_Code=exp for $12.
Now you need a 12-inch wide 1x6 to mount the buttons and joystick to.
The tricky part is figuring out how to attach this to your cab so it is removable, sturdy and Player 2 can see the screen. One idea is to mount brackets to the front or side of your cab for the panel to attach to. Another would be making a "podium" out of the controls so it is free-standing and can be brought out/put away as needed. Or you could just add 3-inch tall 1x6's under each side for a desktop (bartop) style controller.
Hope this helps!!!