You will need an iR unit which is capable of handling what Garmin calls "custom maps". AFAIK, this means a GPSMAP 66i, Montana 7x0i, or later. All newer, "normal" (non-iR) handheld GPSrs are also capable of this.
The problem with the Explorer/Explorer+ is that this is a legacy device. It was developed by DeLorme before Garmin acquired DeLorme. These devices still use maps in the old DeLorme format. They do not support Garmin-format cartography, including so-called "custom maps".
At the Garmin sales site, look for the Specs tab for the selected handheld. Under Outdoor Recreation (inexplicably NOT under Maps and Memory), look for Custom Maps Compatible.
