Hi,
External interrupts are available on one or more pins. The 18F6520 seems to have four located on RB0-RB3. Smaller chips may only have one which is "always" on RB0. (Then you have the interrupt on change but lets leave that for now).

INTCON is the interrupt control register (one of them in the 18F6520). Look at the INTCON register in the datasheet, it'll show you what each bit does. In this case the MSB enables all unmasked interrupts and bit 4 enables the INT0 interrupt.

INTCON2.7=0 disables the internal pullup resistors for PortB.

The PBP is only part of the documentation, you must use the datasheet for your particular PIC as well.

Good luck!
/Henrik.