There's many similar posts appearing, posting a possible fix here as this seems the less likely
to be because of making simple errors with file / directory structure etc.
So...
Assuming you have followed the usual addon installation instructions, there's one more thing you
could check:
Open up the <addonname>.toc file in Notepad and save it as
plain text.
Make sure that there's no special characters (appearing as squares or blank lines) before
the ##Interface: line.
If there are remove them and save the file as plain text.
The Addons button
should appear normally if the game can find an addon.
Find an addon = find an addon folder under \Interface\Addons
+ find a .toc file inside that addon folder that has the same name as the folder, and that
it can parse properly!
Example:
\Interface\Addons\FictionalAddon\FictionalAddon.toc
So for starters check the offending addons .toc files and make sure they're not saved as
UTF8 or other exotic encoding, they don't contain special characters (that might escape the parser)
and save them as plain text (copy and paste the
visible text content of the .toc file over it if you have to)
Hope it helps