How to add my own catmod to ct.js (or it not working)
Module's files should be directly inside your zip archive, without subfolders; check if you don't have an extra folder including all the modules' files.
If there were any errors, they will be in JS console — you can right-click ct.js and click "Open DevTools". (Or something. Similar to a Chrome browser.)
Alternatively, you can put your module inside
Can I use npm libraries in catmod?
No, at least not directly. You need to bundle it for browser usage first. On this package's npm page you can see "browser (UMD)" section with the needed script.