On my end, when using NDI Scan Converter, the PorPresenter presentation appears as a virtual input. I can then select that virtual input in a scene by using OBS's "video capture device" in the source window.
1. Right click in the source window
2. Click "Add"
3. Video Capture Device
4. Click OK
5. Uncheck "Use Preset"
6. Select your NDI Scan Converter Source to be your "device" (This MAY appear as a different name for you depending on how you have it set up)
7. Select the desired resolution
8. Set Frame Rate to "Simple FPS Values"
9. Select the desired FPS value (usually 30 or 60 are the standard)
This is what worked for me. If it doesn't appear to your computer as a camera, then we have our NDI's set up a little differently. Hopefully this helps! I know it can be frustrating. If this doesn't work for you then I would suggest watching some YouTube videos.