If there are no quests, you cannot click it. You must add trigger which creates quests by action "Create Quest". Read tutorials from http://world-editor-tutorials.thehelper.net/ if u don't know how to trigger things.
Players must also discover it so it will show i think. It also might be bug in warcraft. Try checking does the button work in other maps. If not, reinstall warcraft.
E: Oh readed agen ur post, if u cannot click the window, i think there is dialog without buttons etc shown, if not, do the same what i sed before edit.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.