Read chapter 3, Language Extensions, in your C51 User's Guide (C51.pdf - accessible via the 'Books' tab in the uVision Project window).
Particularly the section, "Explicitly Declared Memory Types"

Then read Keil App Note 129, "Function Pointers in C51" and go to the Keil site and read the following about using Function Pointers:
http://www.keil.com/support/docs/210.htm

Bear in mind that DATA and IDATA will give you the smallest code & fastest access, but are VERY limited in size;
PDATA is worse (bigger, slower);
XDATA is worst (biggest, slowest).