Problem is I haven't deployed the Yammer for sharepoint wsp correctly.
Instead I tried http://success.yammer.com/integrations/sharepoint/ this link in which select the online option
INSTALLATION::
1 . Download the web part from success.yammer.com.
From Site Actions, click Site Settings. From Galleries, click Solutions. Click Library. Click Solutions. Click Upload. Browse to the Yammer for SharePoint Online wsp file. Click Activate. after which you can easily add web part for yammer
Pre Requisites:: your service account need to configured with yammer credentials hope this would help