You need to elevate the Command Prompt you are using.
Go to Start, type cmd into the 'Search' box, then press Ctrl + Shift + Enter.
If you completed this step correctly, you will now be required to press 'Continue' on a UAC prompt before the Command Prompt will display.
There are numerous cases where the DHCP database can only be restored if you boot the Domain Controller into Directory Services Restore Mode. This enables you to log in as the local server Administrator to import the database. If the above fails, this is what you need to do.
-Matt