unable to install LW
 

I'm unable to install LW1.4b for some reason..
After doubleclicking the "install" icon, there are occuring 2 messages.
The first one is something about MRJ, that it's too old to run the software. But thats not the point.
The second message is an error, it says that Stuffit Expander can't be found.
I don't understand why, because i've got the latest version installed. It works everything else exept LW.

Is there someone who knows what to do?

btw.. i've got a mac running macOS 8.6