I'm trying to save the lightmap, but I'm really confused as to the process. If I have "writable" checked, and don't specify a file, it does the full calculation. If I put a new filename there, it jumps right to the rendering, and when that's done, it says "no such file or directory, can't write". If I don't have "writable" checked, then it doesn't even render, it just errors out.

The pdf you linked me to says "You don’t need to specify an image name. But if you do, you can render the scene once, and uncheck the “writable” button."