Open de mendix console and go to the Database Configuration tab on the left. Make sure the host name is specified and reachable Make sure the database name is spelled correct
You can use integrated security, in that case the account running the service must have access to the SQL server and is database owner (dbo i think?)
Or you can specify a SQL built in account by specifying a username and password. This account also needs to be database owner (dbo i think?)