"this audio unit plugin could not be opened"

I downloaded the sylenth1 demo and I use the 64-bit version of ableton live 9. But sylenth1 didn't show up in ableton because it was a 32-bit, so I changed it into a 64-bit version. After that, sylenth1 showed up in my vst-folder in ableton but when I clicked on it nothing happened and I couldn't use it. Also when I dragged it to the "mixer drop area" (where the tracks are on the right), a screen popped up with "this audio unit plugin could not be opened".

what do I have to do, how can i fix this and use sylenth1 demo?

Thanks, Luc