That is exactly what my example does:

size_t the_address = 0x123456;            //specify a numerical value that
is to be used as an address
long * ptr = (long*) the_address;            //declare a pointer that points
to the specified adress
long the_value = *ptr;                            //read the value that is
stored at that address.

or in shorthand:

long the_value = *(long *) 0x123456;

but you have to use SEH around this to catch possible access violations
because you don't know if you can access that address or not.
This also only works for virtual addresses. if you want to do this with
physical memory there is no choice but to write a kernel driver or use a
kernel debugger.