How to Install Extensions in Google Wave
When going through waves, you must have come across certain waves that use extensions within them.
Any installable extension is displayed within a wave by a puzzle piece that describes what it does. And you can install them by simple clicking the Install button.
Related: How to Add Gadgets in Google Wave
If you want to browse through other extensions, you can do that from the Extensions Gallery in your Google Wave account.
You can either search your account for Extensions Gallery, or you click the Google Wave extensions link in your Welcome Wave (also available via search). From there, you can browse through and install a variety of different extensions.