First open the package manager to install the open jdk.
Type 'jdk' in the quick search and select the package named 'openjdk-6-jdk'. Apply and finish the install.
Next download the eap by doing a quick google search for 'RubyMine eap'. Or you could use the current eap found here.
Once this is downloaded extract the contents using tar xvzf download.tar.gz. After this is complete cd into that extracted directory. And cd again into the bin folder. Next you simply type the below to start RubyMine
That's it - no extra work required. It is important to note that these eap downloads expire after a short time so if you like the product support jet brains and buy a license.