π
The "Ruby Console" is 'built-in' to SketchUp, it's always available... so use:
Sketchup Menu > Window > **Ruby Console**...
BUT, before asking such simple questions, please try to learn to use some of the myriad of Help files and search options that Sketchup offers - it clearly explains this particular issue here, when used with a with an search for 'open Ruby Console'...
http://support.google.com/sketchup/bin/search.py?hl=en&ctx=en%3Asearchbox&query=open+Ruby+Console&oq=open+Ruby+Console&gs_l=partner.3...1565.1899.0.2148.3.3.0.0.0.0.63.179.3.3.0.gsnos%2Cn%3D13..0.0.313j48989j3j2..1ac.1.YGB9iEqjChI
We're here to help on the more awkward stuff π