vst folder in Cubase 4.5

I want to install a vst plug “Replacer”. The instructions indicate to simply place the replacer.dll file in the Cubase vst folder. Which folder and where is this folder specifically located?

At one point some last year, I had it working, but now when the plugin gui opens, I can’t select any of the enable or disable controls for the plugin, however I can raise the level controls up and down. Consequently it doesn’t process the selected track in any way.

The other question I have is about the preview rehearse function in the plugin gui, when I select this for any plugin, there isn’t any audio to monitor the effect.

I am using an external mixing desk and 3) 8 channel ada8000 converters for a total of 24 tracks. I don’t use the cubase software mixer for anything. How do I route the preview / rehearse audio to a channel on the ada8000?