it doesnt show until you have 100 posts and you get rep by being given it by other members and also for certain amounts of posts..... your karma (which is what boosts you rep) is in 'your profile' and you give and take karma/ rep (when you have some) by clicking on the scales under the persons name..... you can leave karma comments before you get to 100 posts but wont affect the persons rep
red=bad karma
green=good karma
bluey grey= giver/taker didnt have enough to affect it......
thanks for the karma danny, i forgot to mention that you have to write a comment or the person you gave it t wont know who its from, i only know it was you because its blue/grey....