To the left of your window screen, there is a "window" showing your name, title & possible avatar....down at the bottom of that window you'll see a "ying yang" symbol - which is the karma button. Click on it & another window will pop up with "I approve" (green/good) and "I don't approve" (red/bad) options along with a space to type a message if you wish.
Karma is usually used for quality posts, helpful people etc while red karma is given to spammers, people looking for sources, etc
As a platinum member, by giving a Non-plat member red/bad karma, that will also "bomb" them - a small time bomb will show up where their green-dotted karma is. 4 of those bombs = a blackout. You can also go to the karma store & purchase a G-bomb, diffuse bombs, give karma, etc.
Now, you are not allowed to solicit for karma (ie GIMME GIMME GIMME) or start karma threads (ie whoring), etc......