Adding a Gift Certificate to an order manually?

I have searched the wiki on this and can't find an answer.

A customer returned something and exchanged it for a lower priced item. I'm giving them store credit for the remainder of the purchase amount. I created a gift certificate product for the correct amount of the store credit and I have a code to send them so that they can use it. 

Now what I need to do is add this gift certificate on their order, but I have been unable to do so, although I am given the option of adding it when I try - it just never actually does it.

 

Here's what I am trying to do:

On the edit order page I click "add product to order"

I go to my "gift certificates" category

I select the gift certificate that I just made and click "add to order"

I get a yellow box on the next page that is obviously supposed to have some message in it, but it's empty.

I am not able to add the gift certificate to the order. 

Does anyone know how to make this work? 

THANKS!