Yeah I agree with the other posters, that you need to have the vet show you where to administer the shots.
Also, if you want to still try orally, I have heard of a trick that might help. If the horse already anticipates the medicine to be bitter or unpleasant, you could take the dropper and fill it with applesauce instead, giving it a couple of times a day so that the horse will associate it with a pleasant experience. That way, when you actually go to give them the medicine, they won't expect the bitter taste.
It's kind of tricky and might make you feel like a jerk, but I have heard that this works if a horse is giving trouble taking medicine.
Good luck to you and hope your horse feels better soon!