This problem is seen on both 1.8.0 and 1.8.1-SNAPSHOT. I have tried it on both OpenSuse Linux and Windows 2008 R2. Both have the same problem.
According to the Events List page, to acknowledge and event, I have check the Ack check box then click on the Acknowledge Selected Events button.
But that doesn't happen. After clicking on the check box, only the Bookmark Result button appear at the bottom.