i don't quite know the reason for this but its happened to me before.
iv'e gone onto someone's server tried to make an event (like a trap door, or something) gotten out the wrench, hit the brick and opened the events page and the addevent button wont appear. sometimes it just takes a while but then again it sometimes doesn't work at all.
it might be a glitch in v10, but it doesn't happen off line though