1. Create a user on both Alfresco and Liferay.
2. Make sure the user has read/write privileges on Alfresco's site/doc lib.
3. Change the login to "screen name" on Liferay.
4. Configure parameters in your portal-ext.properties file[1].
5. Restart Liferay
6. Go To Document Library and Media portlet in Liferay's control panel
7. Add CMIS repository for Alfresco using AtomPub.
8. Provide the atompub URL. Example: http://localhost:8888/alfresco/service/api/cmis

[1] portal-ext.properties

    atom.servlet.hosts.allowed=
    session.store.password=true
    company.security.auth.type=screenName