The Cadd9 chord is a C major chord with an additional 9th D. Therefore, it consists of the tones C (1), E (3), G (5) and D (9).
Compared to a C9 chord the Cadd9 has NO 7th. C9 in contrast is just an abbreviation for C7(9).
On the guitar you can deduce the Cadd9 by taking a plain C major chord and searching for a second root C on one of the higher strings.
You can abandon that unneeded C and slide two frets (1 whole-step) up to D. If you started from a barre chord, this may be a little bit painful in the beginning, since you have to strech your fingers over five frets!
Although it doesn't actually play a role for the chords name in which octave the additional D is played, it must be notated as add9 instead of add2. The numbers up to 7 are reserved for the chord tones and their modifications. In a sus2 chord for example the 3rd is abandoned and replaced by a 2nd. Actually the 2 only occurs in sus2 chords. Nevertheless, If you stumble upon a Cadd2 chord (written by somebody who didn't take it too serious), you can read it as Cadd9 chord and have mercy with the composer!
Add9 chords have a very shiny, positive sound. You can theoretically replace any plain major chord with an add9 chord, if the spirit of the music allows that positive character.
It is recommended to put add9 into brackets: just compare the readability of Cadd9 and C(add9) in different keys, especially with b-accidentals like e.g. in an Abadd9 chord.