Alas disabling SIP is a step too far for most people.
And there are rumours that the "SIP Disable" feature will be removed when 10.11 is released later this month.
I'm sticking to Mavericks on my machine at the moment. Mind you, I was still running OS 9 + FinderPop on my iMac in 2005!
On El Capitan, FP does NOT work when you control-click something in the Finder. Or rather, it "works" too well - the Finder's contextual menu items do not show up in the menu.
I will hopefully have a bit of time this weekend to work on this, fingers crossed I can get it working. But again, the need to disable SIP is a big no-no for all but the most die-hard of users.