I appreciate the swift, detailed response, but I feel a little foolish here, sorry.
"... Secure browser is set to block installation of all non-avast extensions, just untick block and accept risk."
I don't see the option to unblock anywhere. There's nothing to unblock at the bottom of the avast addons page where it says "Go to Chrome webstore". When go to the Chrome Webstore there's no option to unblock. When I put in an extension I get this
Except Chrome is already installed.
Would you mind explaining where I'm going wrong? Other than owning a computer in the first place...