Looks like this is the syntax. Can't test. Don't have any named instances around.
If you're using windows authentication
sqlcmd -S servername\instancename -E -A
If you're using SQL authentication
sqlcmd -S servername\instancename -U username -P password -A
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability