You'll find this setting on a different place:
Open the DNS-Tab in the UMC and click on the zone-file-entry in the right table.
For expample, if you want to set a @-record for my.domain, you click on my.domain in the table.
Then you will get the settings-dialog with the headline DNS: my.domain.
Navigate to IP-address on the left side.
Here you have to set the IP-address, wich should be your @-record.
Save and your done.
If you want to create a @-record for a subdomain, you first have to create a new forward-zone and then use the steps above.