OS X Automator: Using custom command in AppleScript's "do shell script"
What's the fully qualified pathname of the executable?  If it's not within in the $PATH that's passed to the do shell script command, which is /usr/bin:/bin:/usr/sbin:/sbin, then you'll need to use the fully qualified pathname of the executable.

To make an executable available globally, it must be in a directory that's included in the $PATH.

You can also add an alias or symbolic link to the target executable to a directory that's in the $PATH if you do not want to either append the path to it in the $PATH however keep in mind if the executable is dependent on files that are within its directory then adding the directory path to the $PATH is what you'll need to do.