Use the ! key to flag a post. In Firefox, it is possible to tab through and activate options as one would expect.
In Chrome (Windows 7 & 8) and Chromium (Ubuntu 15.04), tab goes from “off-topic” > the community guidelines link > Spam - then it goes to the browser URL bar, and the last two options are unreachable. “Inappropriate” is only reachable by using ShiftTab from the community guidelines link.
Happens on our instance, and here on Meta.