It sounds like you are using a 2 handed weapon. Try un-equipping your weapon or switching to a 1 handed one, and the shield should be enabled.
<img src="/ubbthreads/images/graemlins/kitty.gif" alt="" /> Yes, that's what the problem was! Thank you Raze. <img src="/ubbthreads/images/graemlins/kissyou.gif" alt="" />