I knocked out the plug in a garbage disposal but it's not hooked up to a dishwasher. Now what?

Get a short piece of 5/8 hose, (about 2 or 3") and a 1/2" copper pipe cap and two clamps.
Put the pipe cap (outside diameter is 5/8") in one end and secure with clamp. Put hose on outlet of disposal with other clamp.

OR

Stuff a wad of paper towel in the drain outlet about 1", fill the remainder with hot glue.

Do as Harold said. For temporary plug, you can take small ziplock plastic bag, wrap it tight around the inlet and put rubber band over it. That will prevent water from spilling out.

Hope that helps

Milo