Although I don't know much of String Theory, what I understand is as follows:
Gravitons are hypothetical particles associated to gravity. String theory predicts the existence of these particles as vibrating closed strings. M-theory deals with the so-called "D-branes", which are models for our universe(s?), and gravitons, which are not "stuck" to these D-branes can leak out to other D-branes, thus causing weakness in gravitational force. See http://en.wikipedia.org/wiki/Brane_cosmology
As far as analogues to magnetic poles, electric charge monopoles were recently produced in the lab. But for the other forces, esp. strong and weak nuclear forces, I don't know whether such a concept exists.
I hope a physicist among us can help us out better.