I'm running Logic Pro 9 in 32-bit mode and can't open Sylenth1. When I go to Preferences > Audio Units Manager, it shows up and says it's validated:
But when I go to actually open it, it's not there.
What's going on here?
That's strange - you've apparently worked out that you need to be in 32 bit mode for the AU Manager to scan the installed Audio Units and then tried to load an instrument into an effects insert. That channel strip is currently showing the ES2 synth as its instrument, so click on there and try again.