YouTube DJ Mode works on YouTube watch pages in Chrome (youtube.com/watch). Live streams, Shorts and ads are not supported.
Open a video and the DJ Mode panel appears on the page. It turns the video's timeline into an 18-key cue sampler: map a key to a moment in the video, then press that key on the panel to jump back to it, like the pads of a sampler. Your cues are saved for each video on this device.
If the panel does not appear, reload the YouTube page and check that the extension is enabled in Chrome's extensions page. Export JSON saves a copy of your cues for a video; clearing extension data or removing the extension deletes saved cues.
For help, write to support@wielder.de with the version (1.0.2), your Chrome version and the video link.