There should be a teeny tiny 1.5mm hex head screw that you have to undo first. Be careful using cheap allen keys on this one as it is easy to round out.
Where is it though? The plunger pin screws into a rotating barrel that sits in a housing that is a part of the lever itself. Am I supposed to get the barrel out of the housing and unscrew this 1.5mm screw from it?? Getting that barrel out was one of the things I wasn't sure I needed to do.