mad4themouse is correct. AP's (regular or Premium) from WDW cannot be used at DLR. WDW park hopper days from either old PH/PHP's or MYW tickets can be used. The process is a little cumbersome.
You must take your WDW PH to Guest Relations at DLR where they must phone Guest Relations at WDW to check on the validity of the ticket and to have the admission removed from the ticket record in the WDW computer. Then DLR GR will issue you a one day PH ticket to DLR. If all goes well, it takes about 15 minutes to do.