Open up a terminal (Applications->Accessories), and type this in to install Java:
sudo aptitude install sun-java6-bin
Authenticate yourself, this that and the other. Afterwards, open Firefox, go to the shoddybattle site, download it like you would on any operating system, and then double click it. It should be in your Downloads Folder under Places I'm pretty sure.
This is assuming your system is Ubuntu which is the most common one. If its something like Fedora, type in su first, then yum install sun-java6-bin (not sure).