I installed RVM using the single instruction mentioned at the RVM website (using git).
Then I installed Ruby version 1.9.2 and 1.8.7 using:
rvm insta
Ruby is not in your path. In simple terms, RVM handles the switching of Ruby in your path. Look at the output of the command-line tool
printenv
You should see something similar to:
PATH=/Users/myuser/.rvm/gems/jruby-1.5.6/bin
See Tin Man's response, it should get you were you need to go.