The IceCap replaces the Gyre controller. You use the same power brick (which plugs into the IceCap) and connect the pump up to IceCap directly. The IceCap also plugs into a 0-10V port on the Apex.
BRS has good details:
http://www.bulkreefsupply.com/maxspect-gyre-interface-module-xf150-icecap.html
There's a link to the instructions there if you want to see a picture of how it connects.
Note that if you
know you're going to go this route, you can save some money by getting a stand-alone Gyre (without controller). If you do this, you'll need to find a power brick for the IceCap, but that shouldn't be overly hard, I wouldn't think...