When you configure Alfresco in Salesforce, in the Salesforce Alfresco Setup tab, you configure both Site to Object Mapping and Metadata Mapping. When you first set this up, you should see the Alfresco login screen.
If you see a grey Salesforce box in either of these sections instead of the Alfresco login screen, this is because you have a browser error, which is caused by a self signed certificate not being approved by the browser.
You should also check that your Salesforce and Alfresco instances are on a shared network or otherwise accessible to one another to share information.