Sorry - I don't quite understand what the procedure was that you were being tested on.
In the simulation, did you have an emergency in progress? If not, you should NEVER use the word "Mayday". Sure, it will get the controller's attention, but unless you are in dire need of urgent assistance, misuse will result in a later phone call to explain your actions.
If you did not have an emergency, then perhaps the controller (examiner) was trying to hint that you were on the wrong frequency? Or maybe he was just being difficult to see how you handled it? Or maybe he's an idiot who doesn't know what he's doing (unlikely)?
But use of the word "Mayday" is detailed in CAP 413:-
"Mayday, mayday, mayday
[Station called] If appropriate - leave out if calling on 121.5
[Callsign]
[Nature of the emergency]
[Intentions]
[Position]
[Pilot's qualifications] Particularly if a tyro or student
[Souls on board] So they know how many bodies to look for!"
Any later calls, you would leave out the word "Mayday" and make calls as per normal procedure. It is down to ATC to keep the frequency free for you, or they may offer you another frequency if that is difficult for them. You are, of course, entitled to refuse if your brain is full and you're working hard enough without the extra bother. Remember that, in an emergency, it is YOU calling the shots - they're there to give you anything you need.