If you have trouble installing a key fob, first ensure that it is has not already been paired with the Link. To do this, go to the "System" menu, and verify that it is not listed.
Next, verify that the key fob is within radio range of the Link. To do this, try to install the key fob just a few yards from the Link. Go to the "System" menu, tap the "+" sign in the upper right corner, and follow the prompts.
Finally, make sure there is not a problem with the battery by restarting the operation using a brand new battery.