This is exactly the problem I have and the same version of SoapUI that I am running (5.2.1). I was overjoyed to find this post, but I cannot get the solution to work for me.
When I slavishly added the line
log.info(mockRequest.getRequestContent())
to the "OnRequest Script" box in my REST MockService editor, first of all it seemed to object to the syntax by highlighting one of the brackets. So I added a semi-colon at the end to make it read as follows:-
log.info(mockRequest.getRequestContent());
The highlighting disappeared, so I figured that was a good move.
When I ran my mock service and threw a request at it, still I was unable to inspect the request contents wit the double-click on the entry in the Message Log.
Am I being daft here? Is the word "mockRequest" a key word or do I have to replace it with the specific name of my request (in which case which name would that be)?
Also, I have a "Message Log" exposed in my running mock service; where is the "Script Log" which you mention? If you mean the "script log" button at the foot of the SoapUI IDE, this just shows entries which look like this:-
Fri Feb 16 17:39:28 GMT 2018:INFO:null
The "http log" appears to record, in a very raw form, the incoming request; but it does this whether or not I include this recommended line of script code above.
So I am still baffled.