You will need to download and install 3 main items:
1. Java Runtime Environment (JRE)
2. Vassal Engine
2. DnD skirmish module for the vassal engine
Vassal is the main application and many modules can be added to play lots of games including of StarWars minis and Axis and Allies and many out of print board games. You play in real time over a live Internet connection.
Java may already be installed on your computer but it can be difficult to tell. So the easiest method is to get the latest version from Sun Microsystems below. It will detect your system and dowload the correct version.
There is a main module and an EC extension. EC extension is not essential and can be added later.
The main module is a .zip file which needs to be extracted. If you don’t have a unzipping program you can try here: http://www.winzip.com The evaluation version is free.
Now you should be able to open up the Vassal program.
It wil bring up a small box with various options. Choose play module.
From the suggested list, load the DDM Mod file that you downloaded and the Vassal program should run from there.
Once loaded, you need to connect to the server to actually see and play opponents.
The program window has various icons and a Help menu which explains functions.
Much more help here as well at Max
Minis. vassal help on max minis
Now find an opponent!