With your MAMP there should be a www folder somewhere in it. As you are familiar with html I woul first figure out the MAMP setup with a simple html page

If similar to a PC based WAMP - you create a folder inside the www like "test" and place your html file inside it. In your browser you enter the path to the MAMP install(PC is localhost//www/???) and it should load your page.

Once you get this working for html then create a new folder in the www area for zen cart and copy the distribution files for one version to that folder(after expandng) and in the browser enter what you have learned from your html experience but change the last folder to that of Zen Cart and append zc_install.

This should start the installation process