Unable to install third party extensions

Hey everyone! I've ran into some problems when trying to install third party addons. In the past I used to be able to do that (couple of month ago) but now nothing.

First I download the extension, click it, Safari pops up the the window asking me if I really want to add the extension. I click yes, its safe aaaaand nothing happens.
As you can see from the image below I have to third party addons installed already. I've been googling up some solutions and the ones I came across (regarding the permission with the Extensions folder) but none of it could remedy my problem.

Thanks in advance!

Cheers,

Khemist