How to dim (and un-dim) display in automator
Where did you install the “brightness” command? Even if you've set up your PATH in the shell rc scripts, it may not take effect for these commands because Automator isn't run from a shell. (You can set environment for all programs including GUI using ~/.MacOSX/environment.plist, but I wouldn't recommend fiddling with PATH just for this.)

I suggest changing your Automator workflow to use the full path to where it's installed, e.g.

/usr/local/bin/brightness 1