Bash gives me a 'bash: unar: command not found' response. Though I triplechecked that the unary executable is in the same folder.
When I run a terminal I get the same response, though clicking on the unar file itself gives me the 'help' response.
It sounds like "." is not in the PATH. Try "./unar" at the terminal. The fact that clicking the file executes it means that the permissions are fine. The error you're getting means that the system cannot find the file in the normal places. Which means that the current directory is not one of the places it's looking (which is a fairly normal setup for security reasons).
When you use the Shell in your app, it's best to use full path names instead of relying on relative paths. The shell does not inherit the full environment; it's your responsibility to set up the environment as needed for any program you wish to run. This is especially true of the "curent working directory". Don't assume it's the same as your app's.