Some MacOS user will notice that after updated to El Capitan and Safari 9, installing Safari extention seems not working like before. "Install now" is no longer exist, it was replaced by "Trust". However, when click the "Trust" button to install external extension, extension will not be installed in to Safari. That has nothing to do with the extension compatible issue. This seems like a bug on the "Trust" button.
Mouse click on the "Trust" button is ignored. However, there is a trick to get this work. We can use keyboard to confirm the installation. Before that, we need to enable the keyboard control access in Windows and dialogs. For OS X 10.11.* or maybe later version if bug still exist. Go to System Preferences > Keyboard > Shortcuts > Full Keyboard Access, select "All controls". Now we can press Tab key to move focus between button in Windows and dialogs.
Now try to install Safari extension, when the dialog pop up, do not use mouse to click the "Trust" button since it will not work, use keyboard instead. Press the "tab" key to move the focus to "Trust" button and hit space or return. Extension will be installed.