Platform and environment
I've found that ruby-processing runs just fine with both Oracle java 7 and OpenJDK 7 on linux, however the processing guys are not convinced, and include a doctored version of the Oracle jre (jdk for the Mac) in their distribution since processing-2.1.0 (I don't think you can use this for ruby-processing). You should have at a minimum a java 6 jre installed for the Mac, and java 7 Windows and Linux. The adventurous might be interested to know Oracle java 8 can also be used (since processing-2.1), and performance improvements have been seen.
You can install ruby-processing using MRI ruby version 1.9.3+ (but should consider installing the latest jruby to use gems with ruby-processing).
Read processing-2.2 platform requirements according to the processing developers "here":http://wiki.processing.org/w/Supported_Platforms. This is particularly import for Mac and Windows users. However you do need a relatively decent graphics card to run sketches in P2D, or P3D mode (ideally the graphics card should support OPENGL 3.0+, we test for this during install on linux and Mac, which is used under the hood by both P2D and P3D since processing-2.0)
Ideally you will have wget installed, but you can get by without it you should have a recent installed version of vanilla processsing.