15 PRODUCTS selection

I'm trying to figure out how to select the products I want for the "15 PRODUCTS" sale, but I don't know how it works.
When I put the 15 PRODUCTS in my shopping cart it's just there, but I can't select the products yet.

How does it work?

At checkout you type the products in the comments box, that's what I did. e.g. if you got 3x50ml of test e then just write that in.