Since the header file declares ADCCON1 as being a SFR at address 0xb4, you can access ADCCON1 by name and just assign a value to it.

#include <CC2430io.h> //or whatever the actual name is!

ADCCON1 = 0x73;

You really need to read the IAR manual to understand the embedded side of things. The SFRs are described on page 162 of the c/c++ compiler reference guide. I would also suggest you read the chapters on efficient programming of embedded systems also.