Fully exiting Opus (via File > Exit Directory Opus) and then re-launching it may make it see the change.
It shouldn't be needed though, and works OK here from a quick test on both Opus 11 beta + Windows 7 and Opus 10 + Windows 8.1. In both cases I made a new library, then added an empty folder to it that I'd created below the Desktop folder, and Opus saw the new folder immediately.
The only thing I can think of that might cause Opus to not be notified of the change is if dopus.exe has been run as administrator via UAC (which can cause a few problems and should not be necessary).