Click the thumbs up button at the bottom of the post next to the multi quote button. Then click the "add to reputation" button in the window that pops up. If you can't do these things, ask an admin for assistance.
You might experience this issue if you're using the app on an iPad or other such device. Try using a laptop/desktop instead.
Edited by Jaimee - 7/30/12 at 6:32am