I know this sounds silly since most people have only one Garmin account. But are you sure that both GCM and Explore are signed into the same account?
IIRC, if GCM knows about a supported device that Explore does not know about, you will see that device on the Pair Device screen in Explore. You can tap it right there to add it. If there is no such device, you will see just the Register Device with Garmin Connect "button", which opens GCM so you can add a device there. Note that the "register device" button appears no matter what.
It also appears that support for the 945 was added relatively recently. You might check to be sure that you have the most recent version of the Explore app from the app store.