You move into the second octave.
So a Cmaj7 (add 9) chord would be:
C E G B D
The C7 (add 9) chord is:
C E G Bb D
Look at it this way:
C D
E F
G A
B C
D E
F G
A
Where the bold tones are your chord tones (I III V VII IX XI XIII).
Basically, to add all the chord tones in one chord and stacking them in thirds, you'll have:
C E G B D F A
Which on paper is a Cmaj 7 (add 9, 11, 13). Pretty big chord!!!
If you see a C9 chord, generally you use the major triad (no 7th) and add the 9th, so you get:
C E G D (C E G is your major triad, D is your 9th)
C11 (C major triad and the 11th scale degree) - C E G F
C13 (C major triad adding the 13th scale degree) - C E G A (the A must be up the octave otherwise you'll play a C6 chord. But they're basically the same chord)
So for chord construction in C major by thirds (which we do most of the time) you have these chord tones, in order (ascending):
C I
E III
G V
B VII
D IX
F XI
A XIII
There's a bit more to it, but for now that'll keep you busy