Lpc2000

I am trying to get my LPC2292 to use indirect memory addressing on the
address bus with cs3. It seems ok on the scope but to acutally use it
I have to be able to use the base address and add to it. If I use:

(REGISTER_OFFSET)  = 0x00000055;  //test cs3

the address 80000000 gets the 55 but what I want to do is
(REGISTER_OFFSET + 3)  = 0x00000055;  //test cs3
which I thought was the same thing except 3 addresses up. This always
generates an IAR compiler error. "(Error[Pe137]: expression must be a
modifiable lvalue"
 
 If I try to convert the REGISTER_OFFSET to a pointer in memory it
wont let me establish a pointer. I tried  "unsigned long * ptr1; "and
the same with * char, and others but its a no-go. 


So if anyone would be so kind as to tell me how to solve this, then at
least one person in the world is happy.