Many garbage disposals come with a plastic "knock-out" plug in the connection for the dishwasher drain hose. In my recent kitchen remodel, the plumber forgot to remove this when he installed the disposal and dishwasher - this resulted in water spewing out of the "air-gap" on the countertop the first time the dishwasher was operated.
To remove the knock-out (at least on the brand I have), disconnect the drain hose and punch the plug inwards with a screw driver and hammer. Be sure to retreive the plug from inside the disposal, so it won't damage the grinding blades.
You should probably check with the manufacturer for your disposal brand to see if the same technique is applicable.