Can't install "ClickToFlash" or "ClickToPlugin"

Hardware: late-2012 Mac Mini
OS: El Capitan 10.11.4 (SIP is DISabled)
Safari 9.1 (only user-installed extension is AdBlock 2.54)

I'm trying to install either "ClickToFlash" or "ClickToPlugin", and neither one will install.

What I'm doing:
- download either extension to the desktop
- double-click the ".safariextz" file to launch installer
- dialog box appears: "Are you sure you want to install the extension "ClickToPlugin" ?
- I click the "Trust" button...and...
- nothing.
At this point, the Safari "extensions" pref pane is displayed, but nothing new has been added.

What am I doing wrong?

RESOLVED!

A little more investigating found the solution.

These extensions won't install UNLESS you reboot into Safe Mode first.

Don't know why that is, but it is what it is.

ALSO...

I previously had Safari Technology Preview installed.

After booting to Safe Mode, I double-clicked the extensions, and... instead of installing into Safari, they installed to Safari Technology Preview instead.

So... to get them to install into Safari, I had to first delete Safari Technology Preview.

And I thought things like this were supposed to be EASY on the Mac.
Ha!

And one more thing:
I tried to mark this thread as "resolved", but could not find the controls by which to do so.
How do you do it?