i don't know if yours is covered with coralline, if so, the seam of the screen runs from top to bottom (that is suction cups facing down) and it's outer edge is close to the discharge.
i usually just grab ahold of it by each end and kind of do a 'snap in two' action. if it is too covered with crud, i will get a knife to wedge it open.
then inside of the screen there is a round intake orifice where the cap needs to be turned about 90 degrees and then lifted out.
I think Bierboy is having problem getting the impeller out (after the cover is opened)... if my understanding is correct, and you can pull the impeller out of it well, your best bet is to heat some vinegar solution in microwave, and let the whole powerhead soak in it for a few hours. This will dissolve most of the calcium deposite away, and hopefully let you free the impeller out.
