The Aruba Configuration tab under Device Setup is for campus controller / access points, not IAPs. IAPs only adhere to the Group -> select group where instant APs are in -> template. Instant APs will get a tab similar to Aruba Config in AMP 7.7.
For templates, the variables are listed along the right hand side. There are about a dozen built in variables, and then another 10 "custom variables" which you will sub in the template as %custom_variable_1%. Then on the VC's manage page, you will update custom_variable_1 to the desired setting. Typically, I skip the first custom variable since it allows a larger text in case I need to add a larger template change. If you assign a custom variable in the template, all VCs in the group must have a value for that value on their manage page otherwise a mismatch will be reported.