Hi,
Have you looked at the examples provided by Darrel on his site where you downloaded the DT-Ints files?
May I suggest you use this one as a starting point. The example lets TMR1 free-run so the interrupt triggers at a given frequency (each time TMR1 overflows).

To change the frequency you reload the timer with a specific value each interrupt, so that it starts from a value other than zero.

See if you can get that example to work as is for starters.

/Henrik.