There can only be 10 karma (green) pinned threads at a time. And I'm not sure how long a karma-pinned thread is pinned; I think it may be 6 hours.

And it takes 10 of your karma points away when you karma pin. If you click on your own karma, it will tell you how many points you have to redeem. Click on that number, and it will show you a list of prizes available. (One global thread pin takes away 10 of your karma points). It says click to redeem by the prizes. Follow the steps. Then I always have another window open of the thread I'd like to pin, refresh the first page of the thread, and on the left hand side of the page under the users name who made the first post, it will say "pin using karma points".

Hope you can understand all that. Let me know if you have a problem and I'll help you through it.