Tylan 50 dosage given orally is 0.25ml per pound or 1.25ml for a 5 pound hen three times a day for 5 days. Be careful not to aspirate your hen, it might be best to give her 1/4ml at a time using a syringe without a needle.
Tylan injectable cant be mixed in water. You can purchase Tylan soluble powder but will most likely require a prescription.
Since you dont know for sure if your hen has MG and other birds arnt showing symptoms, you might consider if it's something in the environment causing similar symptoms, ie...