Please, do the following steps:
1. Go to the Newsletters module
2. Click on the "Create a newsletter" button
3. Fill in mandatory fields and press "Next: Write content" button
4. Click on the "Unsubscribe link" link
The Unsubscribe link appears in the content area. See attachment.
If you want to view how unsubscribe link appears in the newsletter you can click on the "View in new browser window" link.
We fixed an issue for unsubscribe link in Sitefinity 3.6 SP2. If you don't use this version, we would recommend you to upgrade your project with the latest Sitefinity version (Sitefinity 3.6 SP2, version - 1936)
Hope this helps.
the Telerik team