Simply Drag and Drop the addons from your folder to Construct 2. The program then will display for you a confirmation message before installing the addons. You will need to restart Construct 2 though before you can use any newly installed plugin.
You might encounter some addons that were issued before the launch of the (.c2addon) files. They are mostly distributed in a compressed files (.zip). To install those, you will need to copy them as follows:
<install path>\exporters\html5\plugins for plugins
<install path>\exporters\html5\behaviors for behaviors
<install path>\effects for effects