The drain hole is located in the pan under the evaporator coils. You have to remove the rear panel in the freezer to access it.
Defrost it using a hair dryer. Run some hot water, using a turkey baster, through the drain to make sure it' clean.
Pull the refrigerator out of the wall and clean the other end of the drain.
Here are the breakdown diagrams and Parts for Whirlpool ET1MHKXMT02 Refrigerator - AppliancePartsPros.com