1. Navigate to mapjs folder, which is where files have been moved to, to simplify merging with argmap repo.
2. Run npm install to grab the dependencies.
3. Run npm run pack-js -- --env.input_map=[path to map.json file] to package MAPJS and all the dependencies into a single JS file which displays the chosen map.json file as a mindmap. This .json file could be a mindmup export or an output from argmap2mup (see s6mike/argmap: Tools for working with argument maps represented in YAML - forked bugfixes and customisation).
4. Open index.html in your browser.

Check out src/start.js to see how the page is wired up and initialised.

For more information about the structure of the .mup/.json output format, see: Data Format · mindmup/mapjs Wiki.