Note that each plugin and behavior has its own subfolder in the above directories; make sure you copy the addon files to their own subfolder like the others in that directory. Effects however are simply a .fx and .xml file that all go in the same folder.
Restart Construct 2 and the addons should then appear in the editor.
When installing .c2addon files, Construct 2 will attempt to copy the addons to two places: both the install directory, and the AppData folder for the current user (%appdata%\Construct2). This means even if you don't have permission to write files to the install directory you can still install addons.
If you use Construct 2 portably, addons may disappear if they only exist in the AppData folder on the local computer. To take your addons with you, either take a folder with all your .c2addon files with you, or make sure Construct 2 has permission to write to its install directory when installing the .c2addon files by running it with Administrator privileges.