I have removed DNS server from Plesk 12 and after that 'Ok' and 'Cancel' buttons are missing when trying to create domain.
This issue has been considered as software issue #PPPM-1742 and was fixed in microupdate #6 for Plesk 12: 12.0.18 MU#6
Please consider to update you control panel to latest version.
If update is not acceptable, you could use following workaround:
Create a copy of file add-domain.phtml:
- Replace it with attached add-domain.phtml.