First, the sub-domain must not be fully hosted, check on the manage domain page in the panel. If you find the sub-domain shows fully hosted in the web hosting column then delete the entire sub-domain and start over.
When you add this type of record you should click the DNS link under the parent domainname. for example if you want to add game.domain.com you want to click DNS for domain.com. Scroll down to the second section on the page, titled add a custom DNS record type the name of the sub-domain you want in the first blank and the static IP in value.
Remeber you must wait for this type of DNS change to propagate.[hr]
And yes you are adding an A-record, so leave the dropdown for record type alone.