Hi Samuel,
If your SQL Server instalation is not already configured for mixed mode authentication which allows using either windows authentication or sql users, then you need to enable that. Google it if needed. If SQL server was not already installed the Platform Installer would install it and configure it for mixed mode.
Then since you are logged in as admin on the machine when you connect using windows auth from SQL Management tools, you can create sql user logins. Under the Logins node under the Security node you can set the sa password to something known to you or you can create a new login and make it a sql admin. sa is the default admin user created during installation of sql server.
For platform installer you need to use a sql user who is admin and it will create a sql user for the database that is not an admin but is a dbo (database owner) on the database and tha is the user it will use in the conneciton string. For public hosting like GoDaddy you usually must also use a sql user not windows auth.
Hope that helps,
Joe