To add a link to your Facebook page, go to Links and add a Facebook icon. You can search for "Facebook" in the icons, in each of the different color sets.
Click on the Facebook icon to add it to your Links List. This appears at the bottom and is where you can now assign the link to your Facebook page. Click on the link icon to expand the link box and enter the link to your facebook page:
For example: http://facebook.com/emailsignaturerescue
Replace "emailsignaturerescue" with your own username. You can find out what your username is by going to your Facebook page and looking at the URL path in your browser.
TIP: It's a good idea to use a short URL with your Facebook link. Without a short URL, if your signature is received on a mobile device, and you click the link, it will take you to your Newsfeed in your Facebook App instead. If you use a short link, this tricks the mobile device into opening the link in a browser and bypasses it opening in the Facebook App.
To use short URLs, go to your Profile > Settings and turn on Short URLs either on all links or per link. If using per link, you need to make sure that the short link tickbox is checked, next to the link in the expanded link box. See an example below.