If you go to Tools -> WebServices -> Enable WebServices support you get offered the option to download the soapUI Plugin as well.
When this is complete a popup box says something like "A new plugin has been installed please exit idea" and IDEA shuts down.
You then have to manually open IDEA again yourself.
If you install a new plugin via File -> Settings -> Plugins IDEA will restart automatically.
The same restart functionality should apply for the soapUI plugin as well.
Incidentally, after I manually restarted IDEA, the soapUI plugin was marked as not compatible with this version! (IU-92.24)