Make sure Global Undo is enabled.
For 2.8x and 2.9.
Edit > User preferences > System > Memory & Limits: Global Undo
For 2.7x use CtrlAltU > User preferences > Editing > Undo:
That panel is called the redo panel, because when you change a setting on it, blender actually redoes the previous operator with the new settings.
If blender can't undo the previous operator first, running it again will add it's effect on top of the result of the previous operator (which is not the desired effect at all).
So this panel is disabled when undo is not available.