We’re sorry, but we’re afraid this is a common Mail OS X Mail problem.

Creating an email signature and installing the Xink App doesn't ensure it will appear when composing email messages within an email account.

How to configure an email signature to appear by opening OS X Mail

Click Preferences from the Mail menu, and select Signatures

From the Signatures window, select the corresponding email account in the left pane and, using the Choose Signature drop-down menu, select the email signature you wish to use. 

A preview of the email signature will appear within the right pane.