If you just want to play the game, download the game "client" and double click the .jar file to play!
If you download the file not marked "source", you'll get a .jar file that you can simply double click to run. If you don't have a server to connect to, you'll need to download the server run that before connecting with a client.
Otherwise, you may download the source code for ER. The code may be compiled on the commandline with the javac command.If that is not available on your computer, you may need to download a JDK, probably from Sun's web page.
I strongly recommend that you use Eclipse if you are going to poke around in the code or help develop. This powerful IDE makes everything easier... once you get used to it, that is.
Thanks to Sourceforge for hosting the downloads and cvs repository for Eternal Realms.