Thrush is an infection that goes dormant when oxygen is present. When it grows in a hoof it causes the hoof to be moist, and it produces that nasty black drainage to keep air out. Do your best to place the horse on a dry ground before attempting to apply Equinell™ Hoof and Mane Oil. Use a hoof pick and brush to clean the hoof, then follow the instructions to apply a thin layer of Equinell™ Hoof and Mane Oil to all surfaces.
There is very likely a deep, ragged cleft in the frog; clean this gently (it will be tender) and put half a teaspoon or so into that cleft. You want to fill it without any running out, but the cleft may be very deep.
Repeat the process each week until the hoof is dry and the frog looks (you guessed it) normal, and is comfortable. After that use it every four to six weeks to keep it normal and comfortable.