Lode Runner Online: The Mad Monks' Revenge can be downloaded from the MMR website. Play the game on Windows, MacOS and Linux.
Lode Runner 2 is still in progress. Stay tuned!
If you'd like to edit puzzles outside of the game then you need the excellent Lode Runner Hacker's Editor created by Jim Winkler. You can download a copy for Windows PCs here or via Jason Summers' site. A copy is also available for macOS users (143mb) here.
Thanks to Trevor for creating the macOS version of LodeEdit.