Have just installed WCS 18.104.22.168 as part of a new installation.
Have defined my buildings and loaded floor plans (PNG) however every time I try and use the Map Editor it just hangs at "Loading(%) 0%" and nothing happens.
Have ensured flash is installed.
Yes, this is a bug I filed :
CSCtc41084 WCS map import from file fails and does not provide an error message
The problem is that images over 72 dpi are not yet supported in WCS 6.0. The next wcs release should fix this.
In the meantime, your workaround is to save the image under a different format or the same format but not over 72 dpi resolution.
BTW, feel free to post any WCS questions at the ASK THE EXPERT SECTION and I will answer there.