i would create a domain account if you have not already and use that to scan the db, it will need access to the db to read from what i remeber. look at the 2 links i have as they should cover everything you need
https://www-secure.symantec.com/connect/forums/network-discover-unable-scan-sql-server-database
https://www-secure.symantec.com/connect/forums/symantec-dlp-115-sql-scanning-ms-sql-server-configuration