DIAGNOSIS That leak is almost always because the drain hose isn't seated perfectly in the port, or the little rubber gasket/seal inside the port is missing or damaged.
FIX First, turn off and unplug the unit. Pull out the water tank. Look at the drain port on the back—it's a round opening with internal threads. Unscrew the white plastic connector that came with the unit from your hose. Check that the small rubber O-ring or flat washer is still in place on that connector. Now, hand-tighten the connector firmly into the port on the dehumidifier. You shouldn't need tools; just make sure it's snug. Then attach your drain hose to the connector. The key is that the connector must be screwed all the way in, straight, not cross-threaded.
IF THAT DIDN'T WORK If it still leaks, the internal plastic threading on the dehumidifier's port might be cracked. With the unit off, dry everything and wrap the threads of the plastic connector with a couple of layers of plumber's Teflon tape before screwing it back in. This can help seal minor imperfections.
HEADS UP The drain hose must run downhill from the unit with no kinks or upward loops. Any back pressure from a sagging hose full of water can force it to leak at that connection point.