Nuzleaf requires the use of a Leaf Stone in order to evolve into Shiftry and does not evolve automatically. Nuzleaf is a Pokemon that has already evolved once, from Seedot, which happens at level 14.
The Leaf Stone is available at different places in different Pokemon games. In Pokemon Gold, Silver and Crystal, it can be found on Kanto' Route 25 and can be obtained from Bill's grandfather. In Ruby, Sapphire and Emerald, it can be found on Route 119 or 124 by trading green shards. In Fire Red and Leaf green, it can be obtained in the Safari Zone and in Diamond, Pearl and Platinum, it can be found in Floaroma Meadow. In X and Y, it can be found in Route 8, Laverre City, Route 18 and through Super Training.