It is probably because mod_rewrite is not installed or working on your Apache server.
mod_rewrite is usually installed as part of MAMP but perhaps your magento .htaccess file is not being loaded for some reason.
The first possibility is that if you extracted the archive file on your mac and moved the files using the finder or uploaded them to a mac web server using something like dreamweaver then by default the Mac does not show files starting with a . such as .htaccess and therfore they would not be copied into the web server directory. The solution to this is probably to enable showing hidden files in the finder on the Mac so you can see and copy the files easily. This link shows you how to enable viewing hidden files. http://ianlunn.co.uk/articles/quickly-showhide-hidden-files-mac-os-x-mavericks/
The second possibility is how you have configured your http server. You must tell it to allow .htaccess to override settings in a directory. To do this you must have this in your httpd.conf file.
<Directory "/your/magento/directory/">
Options Indexes MultiViews FollowSymlinks
AllowOverride All
Order allow,deny
Allow from all
</Directory>
Obviously replace "/your/magento/directory/" with the actual path to your Magento installation.
Finally during the installation of Magento 2 when you use the web installation setup in your browser, when you get to the Web Configuration section there is an Advanced Options section where you can turn off Apache Rewrites if you can't get mod rewrite to work. That is fine for experimenting but for an actual web store you would really want to get the re-writes working.
