The Goal Key is like a password to the goal, so it is not displayed on-screen when you login as an instructor. Your local EdReady administrator is supposed to provide you with the key for each goal when it is assigned to you. If you cannot locate this information, please contact your local EdReady administrator for assistance.
The other way to see goal keys is in the student data summary reporting module. Here you can turn on a column that shows the goal key. However, this option is only useful after students have joined the goal.