I am sorry to hear you are worried about Maggie. I am Rebecca, I am a veterinarian, and will do my best to help.
What you describe is most likely a bladder or urinary tract infection (UTI). The urine has a bad smell from the bacteria, and the frequent urination is also typical: the bladder is painful, so as soon as there is even a small amount of urine in the bladder, she feels the need to empty it.
These usually clear up with a course of antibiotics, but you want to be sure she is taking them long enough to clear the entire infection, so I recommend doing a urinalysis before stopping the antibiotics.
Since antibiotics are a prescription medication, you will have to get them from your veterinarian; only a veterinarian who has a "client patient relationship" with you (has seen and examined Maggie) can write you a prescription or sell you a prescription medication.
Cranberry, and D Mannose are supplements that can help clear infection, but not by themselves; usually you would need antibiotics as well. I would get her started as soon as possible; occasionally a bladder infection turns into a kidney infection, and that can be much more serious.
Let me know what else I can help with or answer.