If you're trying out Sencha Touch on a Mac, you might be following their Getting Started guide - that makes sense, perfect place to start.
Or not. It's wrong (as of December 24th 2012).
What you really want to be following to get the command line part work are the 'System Setup' steps listed on the Intro to Sencha Cmd - http://docs.sencha.com/touch/2-1/#/guide/command
Follow those and all will be well (I installed the Cmd to ~/bin so I added it o my path manually, I'm sure you've a better solution to that).