Please Help! Storyline Missing Built-In Variables?

Hi there! I'm attempting to change/show a layer based on what variable is being shown, by increasing/decreasing the variable when the user clicks on a right or left arrow. My issue is that I seem to be missing the option to increase the value of the variable.

This is what shows up for me:

When I KNOW that the "add" variable does exist in other projects:

 

Have I missed a step? Do I need to create the "add" variable somehow?

 

I also have these triggers set up:

 

Thank you!