I had trouble finding the right folder to point to also. I did find it though. I think the problem I had was not clicking the button at the bottom of the box. It would say folder not found at the top of the box.
If you downloaded the off line installer and unzipped it:
1 Install in Waves Central
2 Off line Installer
3 select the unzipped folder (always right click and select)
4 Waves 24.5.18
5 Select Installers
6 cache and V9 will show up on top part of box
at bottom of box it says Folder: Installers
7 click on Select Folder at bottom right of box
8 the plugins will show up in center of Waves Central