HI,
The command:
InstallDB MPPCBPDC\SOPHOS MPPCBPDC SOPHOS50 InstallDB.log
Assumes that there is a Windows security group called: "Sophos DB Admins". I assume MPPCBPDC is the computer name where the SQL instance resides and this is also a DC.
So you should manually create a windows group called "Sophos DB Admins", (domain local) and then run the command.
Similar to:
http://www.sophos.com/en-us/support/knowledgebase/116768.aspx
Regards,
Jak