Depending on the plugin/behavior and the way it has been packaged, the zip might not contain a folder.
To have a working plugin/behavior you need 4 files in a folder.
ex: my C2\exporters\html5\plugins folder after having installed the custom plugin CBHash :
<img src="http://dl.dropbox.com/u/36472942/construct/forumhelp/pluginsfolder.png" border="0" />
And make sure to install plugins in the plugins folder, behaviors in the behaviors one.