How to fix OD diradmin account?
Hope this isn't too late, or that at least it is useful for others.

I tried http://support.apple.com/kb/ht1194 several times to no avail. I was simply unable to authenticate with the diradmin account.

An Apple support person recommended:


Archive the OD from the Server.app → OD panel → action menu → Archive Open Directory Master… 
Select the server in the list and click on the minus button Once the OD has been destroyed, You will see the OD is now Off. 
Create the OD by switching back to ON and create the OD again with the same information as before (including the DirAdmin) 
Create a BackupDirAdmin account with WGM to have a backdoor, just in
case 
In terminal restore the archive you saved in step 1 using 


  sudo slapconfig -restoredb &lt; archive-path > 
  Retest the authentication after the directory has been restored.
  
  Try as I might I couldn't get this to work, and then I noticed that in Mountain Lion server the slapconfig command had lost the -merge option so restoring the database was dumping both the new DirAdmin and backdoor account....


Bizarrely, after a lot of digging around I saw that mkpassdb has a -setadmin flag, so I tried to use this to elevate another slotId to admin rights. This worked, and then I was able to reset the password for the DirAdmin account.