If you have bought Stacks from the Mac App store, you don't need to enter the license and you will not see the License tab.

If you have bought Stacks Professional license from this site after you receive the email with your license, you need to insert it on the License tab in the Preferences

Open the Preferences by clicking the `Cog` icon on the bottom left corner of the window or use the hotkeys Ctrl + , on Windows and Linux and Cmd + , on macOS and paste the license code in the outlined field of the License tab and click the Use license button.