Download JASS Newgen Pack (see useful tools sticky at top of topic list). Then follow the instructions in the Jass Forum, the uppermost pinned topic on how to get Newgen to work.
Newgen allows you to extend the capabilities of the world editor in that way. It is by no means only for JASS users.
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.