Dang, it's written in TS but it looks like I may have missed generating a declaration file or accidentally excluded it from the package when I published. Can you check the node_modules/excalibur-tiled and see if there's a lib/excalibur-tiled.d.ts file? If it was packaged correctly, there should be. Otherwise I will need to fix it.
In the meantime, you can use es2015 import syntax and then make your own declaration file (or build the source and copy it over manually).
Check out the docs, tmx isn't supported, only base64 encoded json.