Unsubscribe links are normally added to the footer of your master version. First of all make sure you have added your footer module to your master version. Click on the link text in the footer - this might be the word 'Unsubscribe' or 'Click Here'. When you click on the words, the link section will open up on the right hand side of your screen. Look for the section that says 'Link URL' and add in your unsubscribe URL. Press save.