I haven't completely run a slient install, but it should go like this:
Mendix-7.0.2-Setup.exe /exenoui /q
The first parameter is for the exe file, while the second is passed to the MSI package inside. Running these on my machine at least doesn't show me any UI. When I ran it with as:
Mendix-7.0.2-Setup.exe /exenoui /exelog ./log.txt /q
I was able to see a log file that indicated the MSI package was called with "/q" and completed successfully.