Sending Variable Value to LMS for Persistence

For technical training, we have created a Storyline template that holds PDF course info and a certification exam. We limit the number of exam attempts to 3 using a variable ("Count") that causes the "Retry Exam" button to disappear after two clicks. Since this is a certification exam and we provide immediate feedback, we set the resume behaviour to "Never".

This works well until the user closes and re-opens the session. Then the variable resets to zero and the user gets three more chances.

I want to communicate the Count variable to the LMS so that the retry count is maintained between sessions. I am following the instructions here:   
http://en-uk.articulate.com/support/storyline/how-to-send-the-value-of-a-variable-to-an-lms

but in Step 2 when I add a trigger to the Short Answer slide, the Value field drop-down does not offer me the Count variable. (It shows only
"TextEntry" and "unassigned" - image attached.) If I select "unassigned" I am still not able to assign the Count variable. What am I doing wrong? 

Thank you,

Debbie