External Oscillator selection is performed from the CONFIGURATION BITS of this PIC.

See section 14 of the Datasheet (and section 3 if you're really bored)... mine's only got 304 pages, so you must have the 'Professionals' copy.

Use your programmer to select HS OSC manually at program time if you are connecting a 12MHz Xtal.

Alternatively, embed the appropriate device CONFIG into your program... see for example...

http://www.picbasic.co.uk/forum/showthread.php?t=543

Finally, don't forget to include DEFINE OSC 12 somewhere at the top of your program to tell PBP that you're running at 12MHz, otherwise it will continue to think you're at 4MHz (the default).