I'm attempting to use the timer 1 ISR for the ADuC814. In assembly I easily use the ORG directive to write to the address that the PC jumps to when the interupt is set. How do I do the equivalent in C? Any help would be appreciated.
You're trying to write assembler programs in C!
A better question would be, "How do I write Interrupt Handlers in C51?" This question is answered in the C51 Manual:
http://www.keil.com/support/man/docs/c51/c51_le_interruptfuncs.htm
Note that locating the function at the correct CODE address is necessary but not sufficient to make it an interrupt handler...!