Following the announcement and then the availability for Office 365 Enterprise plan of Yammer (see http://blog.hametbenoit.info/Lists/Posts/Post.aspx?ID=571 to activate Yammer on your Office 365 subscription), you may want to add Yammer on the top navigation bar
To do so, it’s really simple.
Just logon to your Office 365 administration portal and go to the SharePoint administration from the Admin menu or use the direct URL https://<your tenant>-admin.sharepoint.com
Then go to the Settings section from the left side and switch from the SharePoint Newsfeed to Yammer as Enterprise Social Collaboration
Wait a little et voilà Yammer is now displayed on the top navigation bar
If you click on this link, you will be redirected to your enterprise Yammer network