Power items don't work like that. Pokemon don't gain evs from levelling up, although I believe up until gen four, the evs would only be added when a Pokemon levels up. They only gain evs from killing another Pokemon. Each Pokemon has a unique ev yield which they give to their killer. In your case, even if you level up your Unfezant with a rare candy, it won't gain a single ev. You need to go out and poach some other mons or feed it vitamins for it to gain evs. Since Unfezant is in gen five, I do not believe that you need to level up before the evs take effect
Power items work by adding additional evs to when your mon gains one. For example, let's say a Kyurem kills a Charizard. Overrated lizard hands over three spa evs upon its death. If Kyurem has a power lens, it would gain seven evs or if on gen seven onward, eleven evs. This means that power items would grant four additional evs up to gen six and eight from gen seven. Naturally, this is the same for all the other power items. They add a flat amount of evs to the amount normally gained
Refer to this page for which mons you should go on a killing spree on to get a specific ev
https://bulbapedia.bulbagarden.net/wiki/List_of_Pokémon_by_effort_value_yield
