help

Or just in the 'Search' box on the Keil website - as it is the Keil tools that we're specifically talking about!

Or just in the 'Search' box on the Keil website - as it is the Keil tools that we're specifically talking about!

But the first match in Google will point to
http://www.keil.com/dd/docs/c51/philips/p89v51rx2.h
and using Google should require slightly less creativity than to scan the Keil web pages and notice the Search field in the upper right corner.

DON'T SHOUT!

For a start, where did you get your Keil from in the first place?
If it's old, have you tried updating it?

For each supported chip, Keil provides a page in their Device Database which describes that chip - including, among other things, their header file for it:

http://www.keil.com/dd/chip/3711.htm

Note that you could have found this by simply putting "P89V51Rx2.H" into the 'Search' box at the top-right of every page on this site.

The header file is just a standaed 'C' header file - it doesn't need any special "installation".

You will, of course, have to put it somewhere where the compiler will be looking to find it; the C51 manual describes how the compiler searches for include files - and how you can specify additional locations for it to search...