Did you install the custom behavior/plugin in the right directory?
<drive letter> (if not C:/<programs directory>/Construct 2/exporters/html5/
From here, you have two different folders:
If it's a behavior (the thread will say something like it'll show up in the default add behavior dialog), it goes in behaviors. Otherwise, plugins.
From there, make a new folder in the sub-directory (plugins or behaviors), name it the new plugin or behavior and drop the files into the new folder.