Have you tried it in Cubase 6-32 bit? As far as I have been able to figure out, the MX400 VST is only available as a 32 bit VST. This is a call Lexicon issue, not a Steinberg one. My MX400 VST shows up, but doesn’t work in C 64, but it does work fully and as expected in C 32.
BTW, the hardware works just fine as an external FX in C 64 so you can still use it. It’s only the VST controller that doesn’t work. Meaning, you can’t change the settings from within Cubase. But, you CAN run the MX-Edit software in the background, with Cubase running, too, so you can set it up to your heart’s content. The only problem with that is, of course, Cubase will not remember your settings, so you’ll have to save them manually and remember to set the unit up the way you want each time you load a project and want to use different settings.
Hope this helps.