You must also specify a minimum amount of MSP in the class design window, on the misc tab. Make sure you don't set the minimum too high or you won't be able to transfer any MSP at all.
Ok, the ship carries about 6000 MSP and I tried setting the minimum to 1000, to 10, and to 0. Nothing seems to be different?
You will also need to refit the maintenance ship to the latest design at a shipyard. In your case I would probably set the MSP minimum to somewhere between 500 - 1000 and also verify that you ticked the 'supply ship' box in the design window before refit.
Beware when you do this however. If all you did was change MSP minimum then the refit cost will be zero and I believe the game will throw an error message. So you might want to add an extra fighter fuel tank to the maintenance ship design before you refit your ship so that the refit cost is non-zero.
I have successfully resupplied a ship from another ship before but it is very rare that I do so and I haven't done it in quite a while so my memory is foggy on what exact fleet orders you need to issue. I know that if the fleet is moving then you will need at least 1 level of the 'underway replenishment' tech to successfully resupply another ship from a supply ship. After you get your supply ship to join the fleet you want to resupply you should go check the ship screen in the fleet window and make sure that each ship in the fleet has the correct resupply orders set, including the supply ship. Note the resupply orders are not just a fleet order, there is also a per-ship setting in the fleet window for each ship (similar to how weapons are assigned).