Hi Bill,
Welcome to the forum - and well done on choosing a Phatbox for your S60.
There is no head unit set up in the Volvo firmware. You just plug it in and it should work. I don't have the HU-650, but on the HU-850 that I have, use the CD button to toggle between internal CD and changer (=Phatbox). You select the access mode (playlist, album, artist, genre) using the CD select rotary control. Then the previous/next buttons to navigate. Holding them down in some modes such as artist will walk you through the alphabet reasonably quickly (but that only works on the HU, not on the steering wheel in my case). You then step one at a time to get to the artist that you want.
Anything else - yes. Check and double check the wiring before plugging anything in. See my
wiki page (I guess you already did that
)
And don't forget - the first time that you power up, the Phatbox will re-load the new Volvo firmware from the DMS - so you should get the green LED flash about 3 times a second for 2-3 minutes or so. While that is happening, the HU will report a CD Error. But when it's done, the Phatbox should restart itself and then the error should go away. After that it won't need to do it again. Normal start up is in the 5-10 seconds region (a bit quicker than the nav unit).
I've had this running just about three years now in my S80 - I would be lost without it.
Any problems - shout.
Cheers,
Les.