It should be there. I don't know why it's not. I've never heard of this happening before. Perhaps the software didn't install correctly?
You might want to check the ArcGIS Administrator application to see if the www.arcgis.com appears. From All Programs>ArcGIS>ArcGIS Administrator. Click the Advanced button. Then click Manage Portal Connections. Do you see http://www.arcgis.com listed? This is what points ArcMap at arcgis.com.
You may want to contact Esri technical support as they are better equiped to troubleshoot this issue.
Thanks,
Mike