I've not used ADCIN much, but here's some other things to check:

I don't see anywhere in your code that sets the pins as inputs - you might try setting up the TRISIO register as needed.

In the PBP manual it shows an example like this:
"ADCIN 0, B0 ; Read channel 0 to B0"

The 'channel' is selected with a number rather than port pin name - don't know if that makes any difference or not.

Arch