Go to the Panel, click on "Domains" and "Manage" domains. Then choose which domain you want to point to Tumblr (no sub-domain). Then click on the "DNS" link of the domain which you want to point to Tumblr.
Now you get a form:
Name: _________ .yourdomain.com
Value: the Tumblr IP address
Now either leave the input field at "Name" empty and it will point yourdomain.com to Tumblr. Or enter a Sub-Domain like tumblr.yourdomain.com. If you also want to point www.yourdomain.com to Tumblr you need to create an additional A-record for that.
Hope it helps.
Promo-Code: [color=#CC0000] SAVEMONEY97 [/color] - Save [color=#CC0000]$97[/color] on your first year of hosting.