[Download](http://silverstripe.org/download) the latest SilverStripe installer package. Copy the tar.gz file to the 'Document Root' for MAMP - By Default its `/Applications/MAMP/htdocs`.
Don't know what your Document Root is? Open MAMP Click `Preferences -> Apache`.
Extract the tar.gz file to a folder, e.g. `silverstripe/` (you always move the tar.gz file first and not the other way
around as SilverStripe uses a '.htaccess' file which is hidden from OSX so if you move SilverStripe the .htaccess file
Once you have a copy of the required code (by either of the above methods), open your web browser and go to `http://localhost:8888/silverstripe/`. Enter your database details - by default with MAMP its user `root` and password `root` and select your account details. Click "Check Details".