It disappeared late yesterday evening or in the early hours of this morning, sometime around midnight GMT I'd say.
I checked Proboards support at that time but there was no mention of it however, the proboards forum from where I got the original idea is also "missing" the box and they are on a different proboards server. I think it's reasonable to assume it's either a Proboards issue or that the server running part of the add-on code (not Proboards) is down.
Hopefully it will reappear but if not, it will be an integral part of Proboards v. 4.5 which is currently being tested in the Proboards Support Forum.
Edit: I can now see that it's for the reason I've just highlighted. We'll wait a day or two to see if it reappears or if they change the code to point to another server.