You have to uv map it first-

there is a good tutorial here on the site about that.

or... I would suggest mapping it while its still in polys. you can use the automatic mapping for a quick fix, but that creates a bit of a messy set of uv's.

or... There also is a script on this forum called Danny's UV helper ( or something like that )

I like to select groups of faces and planar map them, then arrange them in the uv editor.

good luck!