In your OJS document root, which is probably MAMP/htdocs, you will find a file ‘config.inc.php’. There is a line which starts with tar =. This line needs to reflect the path to the tar executable.
I’m not familiar with where the tar command lives on Mac OSX. If you are able to run the Terminal application, you can type in a command to find the tar command, either which or whereis. For example, you might see something like:
$ which tar
/usr/bin/tar
In this case, you would change the line in config.inc.php to:
tar = /usr/bin/tar