The command file you pass into SQLCMD should not be the stored proc, it should just be the command to execute the proc:

USE databasename

GO

EXEC storedprocname

GO

Since the proc already exists in the database, the above commands will point the connection to the right database and execute the existing proc. Make sure that the account under which SQL Server is running has permissions to read the directory where the input file lives, and you should be good to go.