What does Macbeth want the doctor to do for his wife?
First, Macbeth asks the doctor for his diagnosis of Lady Macbeth's illness, and the doctor tells him that she suffers from "thick-coming fancies" (she's imagining things---remember the sleepwalking scene?). Then Macbeth instructs the doctor to cure his wife of this affliction, but the doctor insists that "the patient must minister to himself." The doctor has no medicine for a mental illness, which is Lady Macbeth's problem.
Earlier the doctor told the lady-in-waiting while they watched Lady Macbeth that she must be suffering from guilt for something she had done, something that she must confess in order to be cleansed of the troubling dreams and sleepwalking.The doctor had said, "More needs she the divine than the physician," meaning she needs a minister more than she needs a doctor.