[JWait]

In admin, click "languages" then in the drop down box choose your language and wait for the page to refresh. You will see a listing of all of your language variables. Change the dropdown next to "Select topic:" to "labels" and then enter "lbl_my_new_button" in the "apply filter" box (just to make sure it doesn't already exist).

At the bottom of the page there is a section where you can define your language variable. It is marked "Add new entry". Put bl_my_new_button in the "Variable:" box and enter what you want displayed in the "Value:" box (note; the Select topic: should be set to labels) Click "add" when you are finished.