Apologies if I'm just being severely dense (I never use add-ons, but this one would be very very useful to me).
I uploaded the files after unzipping locally on my desktop (this gives me an extras and upload directory, as well as the addon-Sedo_GoToTop.xml file) - to the home directory of my Xenforo installation on my server (/var/www/html/bb) - so pretty basic stuff so far.
Then I go to my Xenforo ACP and select add-ons (bb/admin.php?add-ons/) - click Install Add-ons button, and select the xml file (I've tried both from my desktop or giving the full path such as /var/www/html/bb/addon-Sedo_GoToTop.xml).
Either way, I get the following error:
Callback Sedo_GoToTop_Listener_Templates::goToTop is invalid (Invalid Class).
What am I doing wrong?