Installation procedure
First, if you haven't done so already, get the MMNET.zip file here: http://www.mekwars.org/MMNET.zip. Once you've downloaded and saved it to your computer, unzip the files.
You will now have a folder called MegaMekNET. Inside this folder, double-click on the MegaMekNET.bat file (I also suggest creating a shortcut to this file and placing it on your desktop or with your other game shortcuts) to launch MMNet for the first time. You will see this:
Choose a user name and password (both fields are case-sensitive). Do not change the IP Address or ports. You should then see this pop-up:
Click the Yes button to continue. The client will begin downloading updates. Once it has finished updating, the Login screen will appear. Login with the same user name and password you set earlier. You will see this:
Click on the Maximize button to expand the window.