Would I be correct in surmising that you are not familiar with the Keil tools?
In that case the first thing you must do is to get familiar with the Keil tools!
Read the uVision Getting Started guide, and work through the example projects; read the C51 manual, carefully noting how Keil have made their 8051-specific extensions - compare & contrast with the way IAR have done it

http://www.8052.com/forum/read.phtml?id=23279

I am getting tons of Errors like "Address Overflows" and "References made to Erroneous external".

The latter are probably caused by the former - fix the overflows, and you'll probably also fix the bad references!

You probably need to take a careful look at Keil's "Memory Models" & whatever IAR's equivalent may be.

Do you understand the 8051's different memory spaces - what Keil calls DATA, IDATA, PDATA and XDATA?