Hello,
you might not be aware of the recent changes: The Alfresco Explorer has been removed from Alfresco 5.0.b onwards. As such, you can no longer interact with the repository contents from /alfresco. Use the link to "Alfresco Share" to go to the default UI. Although some concepts may bear different names in that UI, be assured that the feature set of Explorer is covered to 99,9 % (some features have not been kept due to lack of relevance / use).
Regards
Axel