Your Seadra will evolve when you trade it with another person while it's holding a dragon scale (there are two in Black/White on Routes 13 & 18 ). Therefore, for you to obtain Kingdra, you must trade a Pokemon with someone for his/her Seadra that is holding a dragon scale.
See related link for more information:
http://www.serebii.net/index2.shtml
It evolves when it is traded while holding the king's rock.
Seadra can never level up to evolve. You have to trade it when it is holding a Dragon Scale item. - Pokemon Guru
After you get one, you trade it to another game when it is holding a Kingsrock.
Seadra is not a dragon type pokemon (it's just water), however its evolution Kingrda is part dragon. You can evolve Seadra by trading it to another game while it is holding a dragonscale. or in pokemon Black and White you can catch wild kingdra by fishing in rippling water (though it is very rare.)
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no it does not evolve on Pokemon black or white
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No.
Well in the pokemon black and white show,(on cartoon network)she said she is not going to evolve axew.So i bet in the game,(pokemon black and white)she is probably not going to evolve axew.So dont worry.But if axew does evolve,then iris lied.