I'm not too sure that you can do this with a standard electronic tuner.you could try goolgling around for a flash tuner or something similar which has alternate tunings.
Alternatively, you can tune it to your other D-string (4th string) (they're an octave apart) or tune the 6th string, fret 7 to the A-string (5th string).
Cheers

When you say 6th string fret 7 do you mean hold the 6th string down at the 7th fret and tune to the 5th string?

Yes, that's it. The seventh fret of the sixth string tuned to D will be an A, which is the note on the fifth string. This is perhaps the easiest way to do it.

The tunings for drop D are (from 6th string to 1st) D-A-D-G-B-E. Great for playing power chords. For example:

D5 000xxx
A5 777xxx
E5 222xxx

The same easy left hand finger shape is kept and moved up or down the neck for different power chords.

Another quick way to tune it is to play the open 6th string (E) and listen to the 12th fret harmonic (put the tip of your finger lightly on the string at the position of the 12th fret - you should hear the harmonic one octave higher than the open note) - then tune this down to the open 4th string (which is a D). Hope that makes sense,
Cheers

Thanks alot!! That was very helpful

this for tropaneboy saying that D tune can't be achieved on the tuner..IT CAN BE ACHIEVED..you can get any pitch you want from a tuner