This a Premium ($) feature.
Technically speaking you cannot print a stand alone answer key.
That being said, there is work around.
Click on Assessments and then My Assessments.
Click the box to the left of the assessment for which you want an answer key. Next, click on the blue dots on the right. From the drop down menu select print.
The result will be a printable version of the assessment.
The answer key is at the very bottom.
Right click you mouse and select print.
Now, select that you only want the pages that contain the key.