I just went through figuring this out. This is what worked for me. You can't share existing folders. To share a folder it first has to be set as public, which, you can ONLY choose while you are CREATING a new folder, which is totally lame. Also, if the new folder you make is in your bookmarks bar, it won't give you the option to set it to public, which is also lame. So, you have to make a new folder, that isn't in the bookmarks bar, set it to public when you create it, then drag all your bookmarks you want to share into it. Considering this is an extension from google I would expect better backwards compatibility and am kind of dissapointed. Hopefully google corrects this, otherwise I suspect most users will just give up on shared bookmarks, thinking it is a feature that doesn't ever work.