how to map subdomain

Dear friends,
I would like to map subdomain on my WP multisite 3.1.3 with GoDaddy.
It works for standard domains, but I need to put a subdomain into service for the first time.

I mapped to in your plugin. Probably, there is an error in GoDaddy domain management. I put there mysub.mydomain to / (root folder), where is my WP multisite is installed .

What setting should be done for this subdomain?

Thanks a lot,