Neos needs to be in your hand, and it'll ask if you wanna summon it by necroshade's effect. Can't be done multiple times unless you get necroshade on your deck again, I'm not too sure about that.
There's something wrong. I think Neos MUST be in your hand when Necroshade is sent to the Graveyard. While the effect should work regardless of when you get Neos in your hand, the game just doesn't let you. Effect ruling states it can be applied on any of your turns if Necroshade's in the Graveyard, but only on YOUR main phase turns, not your opponents. They need to fix that ASAP.
Keep in mind it doesn't give you an EXTRA Normal Summon like a card like Arrivalrivals does. It just lets you Normal Summon without Tribute, in the same way Mausoleum of the Emperor does. So you need one available.