Look at the supplied "Hello World" program - see pages 44-45 of the MCBx51 Manual

Note the advice on p45:

"You can find more information about the Hello ... example in the µVision2 and C51/C251 Getting Started User’s Guide (C:KeilC51hlpGS51.PDF), Chapter 7. Sample Programs."

You really do need to read that uVision Getting Started Guide, and work through the example projects in it!
This will give you a proper introduction to the tools, how they work, and how to use them - rather than just blindly jumping-in at the deep end!