I originally downloaded some presets in the old file format. They didn't work as expected, so I removed them from the Develop Presets folder and imported the xmp version. But LR no longer sees the folder in the Develop Presets folder.
I know I could manually import them using the + button, but I don't want them to appear as User presets - it a specific collection so I want to see them in a folder as such.
Lightroom no longer organises presets by folders on your computer. The groups are stored inside the preset itself. You can drag them into another group in the Presets panel. You cannot create a group in there, so if you want to move them into a new group you will have to use a little trick: create a temporary new preset. When you create a new preset, you can also create a new group. Now move the other presets into this new group. Finally delete the temporary preset again.