The .013 feeler gauge (or .013 paper strip) is needed to check clearance between the clips and the brake rotor. A gap of at least .013 inch (.32 mm) is recommended (enough to slide the paper strip in and out without sticking). Once clearance is confirmed, you can re-install wheel and tighten lugnuts per manufacturer’s specifcation.
If the paper feeler gauge is missing you can cut of a strip from one of the installation instruction cards that you received. That can be used as the feeler gauge and it will allow you to check for clearance.