How to insert an unsubscribe link
You can insert it by selecting UnsubscribeURL from the Merge Tag drop-down menu in the text editor. To display the toolbar, click the space bar on your keyboard and highlight the empty space. It'll be replaced with the merge tag once you've selected it from the drop-down menu.
If you wish to change the text, you can click on the text and press the Edit Link icon:
Custom unsubscribe links are not currently supported, so you must use the unsubscribe link provided by EmailOctopus.