If rbenv is missing the install command, then you need to install ruby-build.
If you have Homebrew, then run:
brew install ruby-build
You can learn more about this in the README in the rbenv project here.
After you have ruby-build, you can run:
rbenv install 1.9.3-p194
… to install Ruby 1.9.3-p194 (if that’s the version you want).
This lack of an install command occurred in Mac OS X Lion 10.7 and Mountain Lion 10.8.