Project.SlidesViewed Variable

I could use some help with variables (I don't use them very often and have a feeling I'm missing the obvious answer to my question...)

My course has a kind of menu slide that the learner returns to several times as they navigate through the course. When they return to that slide after having viewed every slide in the course, I want a layer to appear with additional information. I figured I could trigger that layer by using the Project.SlidesViewed variable, such that the layer shows when that variable = 29 (the total number of slides in the course).

But when I go to create the trigger and click on Conditions, that variable isn't available (even though I can see it in the Variables window). What am I doing wrong??