A.C. unit is draining on the inside of the house.

There is a drainage and it is stucked with dust, mold etc. Just open metal lid on the wall inside your house, where the coil is installed and where you're changing filters, remove air filter, there at bottom you will see the drainage hole (now under water probably). Rent an air compressor from Home Depot or Lowe's, stick the hose to that hole and turn compressor on. This shall blow the drainage through. After that slowly pour 1-2 cups of Chlorox or Tylex in there, let it stand for 15 minutes, then pour some more water and blow with compressed air again, then flush it with water and blow last time.
Do that at least once in two years, or you gonna have same problem again.