I made a blender IK rig for character that I was modeling. The knee bone bends, but the mesh does not. No matter what type of weight painting I do, the mesh doesn't bend, it just deforms and twists. I also tried rotating the pole target around, but that didn't help.
The bones can effect only the vertices position, it looks like your leg has only 4+4 vertices. Try subdividing it (in three different ways):
Add a subsurf modifier (upper in the modifier stack than the armature modifier)
Using the loop ring tool (shortcut Ctrl+R)
Selecting the mesh in edit mode then hitting W> Subdivide
Search the internet for typical mesh geometry for a good deforming leg and copy the topology.