And most of all,how to set the MCU to operate at 12mhz?obviously is not Define osc 12.i tried
as has been discussed thousands of times osc must be in upper case ( OSC) and only tells the compiler what osc frequency to base its timing calculations on.

CONFIG XINST = OFF
CONFIG PLLDIV = 5
CONFIG WDTPS = 512
CONFIG CPUDIV = OSC1
are config word (sometimes refered as fuse ) settings for a particular pic chip , they must match the chip you are using and the particular configuration that you want it to adopt.
read the data sheet .
consult your manual on methods of setting the "config word(s) " for your chip