how do i release the tension on my timing chain? Please help

[MuseR]

I have an S13 with a ka24e and im doing my head gasket, but when i was reassembling i got the head on and torqued down but i cant get the sprocket onto the cam! i know for sure its the tensioner that is holding tension and not giving me enough slack on the chain to get the sprocket on. so can anyone tell me how to release the tension with only the valve cover off? any help is greatly appreciated.