Google has made some breaking changes to the extension system. One key change is the extension packaging. Check out this page for all the details.
Go get them.
As usual you have to be on the dev channel to use extensions and start chrome with the parameter –enable-extensions.
With the current build (18.104.22.168) there is a bug preventing installing extensions by just clicking on the link. You have to download the extension by right clicking the link and choose Save link as… Once downloaded double click on the extension to install it.