AMS will be mounted on the upper Payload Attach Point on S3 Truss of the International Space Station.  A NASA animation below shows how it will be done.

After the docking to the ISS, on Flight Day 4 as the present schedule, the Shuttle arm (SRMS), after removing another payload, the ExPRESS Logistics Carrier 3 (ELC3), will pick AMS up out of the bay and bring it near the station, where the ISS Canadarm 2 (SSRMS) will grab it, roll out to the truss and plop AMS down on its mount point.

(Image & video: NASA/JSC)