MCollective Shell is a separate Agent plugin, it is not packaged with the core MCollective software. Agent plugin code is installed into $libdir/mcollective/agent. See the general plugin install guide. You can get the Agent code from GitHub.
Ohai, and most Chef-related code in general, is no longer compatible with Ruby 1.8. Ohai 7.x will still run on Ruby 1.9.3, but late versions require 2.0+.