1. login as admin of your XMB board and go to Adminstration panel
2. edit HEADER template
3. add this line :
4. search for $searchlink $links $pluglink and add :
immediately after it.
5. save the header template and logout.
you'll see +1 button on all pages adjacent to your links and plugins on top on each page.
Now anyone browsing your site can add '+1' to your threads and other pages(if they have google/gmail account) and promote your board