xcrysden installation
 

I am new user of redhat linux enterprise 5.0. I want to install xcrysden but don't know how to Define the XCRYSDEN _TOPDIR and XCRYSDEN SCRATCH environmental variables. please help

RHEL5.0 is old...if this is a new installation, why don't you install the latest 6.x version? And have you contacted Red Hat support, since you're paying for your Red Hat enterprise subscription, right?? If you're not, you should be using CentOS, since it's 99.x% IDENTICAL to RHEL, but FREE, and provides you with patches/updates that a non-pay-for version of RHEL won't.
These are environment variables, and how you define them will vary, depending on the shell you're using. Chances are it's bash, so you can either type in:
..in a terminal window, then run your program, or you can put those two lines in your .bashrc or your .profile file. Those are in your home directory, so if you type in "vi $HOME/.bashrc", just add those lines to the bottom, save, and exit.