This is a  simple step by step walk through of how to get KMail installed on your Ubuntu 8.04 Machine. I take no responsibility for anything that may or may not happen.

Step 1: Go open the terminal.
Step 2: type in this command: sudo apt-get install kmail.
Step 3: type in your password.
Step 4: when it asks you if you want to continue: type in y and tap enter.
Step 5: Wait for it to install.