D2 requires MySql. Minimum version is 4.1.x
To install D2 on the server side, you don't need to do anything specific. You just need to ensure you have a running MySql server and an accessible database (even empty).
To run D2 you will need a Java 5 JRE installed on your machine. If don't already have one you can download it on Java site.
No special action is required to install D2 on the client side. Just unzip the distribution in the folder of your choice. To launch D2, run the decision-deck executable (decision-deck.exe on Windows, decision-deck on Linux).