Sketched Bend Tool

I haven't fooled with sheet metal for quite a long time, and even when
I did I used it very little. Will someone help me out and tell me why
I cannot get the option to use the "Sketched Bend" tool. What mode do
I need to be in to have the option? I have a very thin model of a
button cover that I extruded and it has several tabs on it that I must
bend up 90 degrees, and no matter what I do I the icon remains greyed
out to me I am using the planar face of the extruded part to sketch my
bend line, and I exit the sketch, and still no option. No option
either before I enter the sketch mode or after.
Exasperated,
David