Once the download completes, locate your downloads folder and double-click the Rest.dmg file.
OS X might warn that the application is downloaded from the internet. Hit "Open".
Once the Rest.dmg opens, a new window will show. Drag the Rest.app icon to the Applications folder.
From your Applications folder, double-click on Rest to launch it.
You will now be prompted for your license key. If you don`t have one, just hit "Continue trial" and you can evaluate Rest for up to 7 days, for free.
That`s it :)