Generally, 9th chords contain both 7th and 9th notes. Add-9th chords, on the hand, usually only have a 9th note added to the standard root-3rd-5th triad. Here's F#add9:
D string = F# (root note)
D string = F# (root note)
G string = A# (major 3rd)
B string = C# (perfect 5th)
High E string = G# (added 9th, same as a 2nd note)
