Try the following:
go to http://www.alfresco.com/cmis
There is a link to the workbench: http://cmis.alfresco.com/opencmis/workbench.jnlp
Use the alfresco cmisatom url: http://cmis.alfresco.com/cmisatom
Login as admin with password admin. This one works and you can even try your local repo.