Obviously, the hardware needs to be correct - or nothing will work!
The hardware details have already been explained here:
http://www.8052.com/forumchat/read.phtml?id=128598

From the software point of view, simply using the MOVX instruction causes the processor to generate the appropriate bus cycle (including all the activities on ALE, Address Bus, /RD, /WR, etc) to read or write external data memory.

In Keil C51^[1], the xdata keyword extension tells the compiler to use MOVX when accessing the variable.

So, in your C51 software, all you need to do is specify the appropriate memory area.

It sounds like you need to go through the Tutorials:

http://www.8052.com/tutorial.phtml

Even though you're writing in 'C', you need to understand how the underlying architecture works - as this very Thread demonstrates!


[1] Note that this is a Keil-specific extension to the standard 'C' language - other compilers are likely to be (slightly) different...