Please try this: Open one of your plugins and click on the preset settings menu dropdown, which is above the preset picklist dropdown, here:
Then select "Save settings as" and a file selector should pop up. Whatever directory it shows, that's the directory you should use. You can see the path to that directory by clicking on the dropdown picklist near the top, here:
Click that and you'll see what directory you should use.
If you continue to have problems, you can also just go to the preset settings menu dropdown, like before, and select "Import settings" and manually load the presets.