Impossible to create the Wither

I was playing in a superflat world to try the new release, the 1.6 pre-release when i tried to create a wither boss, to rename him with a name tag. After several times putting the soul sand and the three wither skeleton skulls, nothing happens. I also try doing the sme in another world but again, nothing happens. If this information could be useful, i have Minecraft Premium and I was playing in the new launcher. I haven't got any mods. Sorry if there are any fails in my text, but i'm Spanish.