it should come already set as 'install mode', once fitted you should push the chain guide towards the tensioner with a screwdriver, this activates the tensioner
if it's already open, it is possible to fit, just be careful not to cross-thread it when pushing it into the head and starting the threads [you will be pushing against the tensioner]
or
you can reset the tensioner back to 'install mode' by turning the tensioner until it holds closed