Add a number means take the standard 1-3-5 chord and add the said number. I've only play add9 chords, so you'd take the standard 1-3-5 and add a 9th.

Note that add2 and add9 are the same thing, but I've never seen add2. I haven't played enough guitar, so I'll leave it at that. (If you look at the Cmajor scale, both the 2nd and 9th are a D)

The 9 chord (ie, C9) is not just 1-3-5 and a 9, but also a flatted 7.

Wikipedia has some answers, and I don't feel qualified to answer the question further. Just know that the different names correspond to which intervals of the chord you're playing.

http://en.wikipedia.org/wiki/Ninth_chord