No need to edit your INC file. Just add a line of assembler to your program:

@ DEVICE HS_OSC

Then add a line of PIC Basic:

DEFINE OSC 20

I don't believe Pic Basic Pro can use 15MHz properly. See page 170 of Compiler manual: "The acceptable oscillator definitions are: . . . 12, 16, 20 . . ." If you use "DEFINE OSC 15", I believe PIC Basic will default to 4MHz, but then you have changed "DEVICE XT_OSC" to "DEVICE HS_OSC", so it may not work at all.