How do you join an incubator course?
are they open yet? I can't find any info.
You can apply to help with a course at http://incubator.duolingo.com/ but they are adding contributors gradually (you must be fluent in the two languages you are applying for in most cases). Alternatively, if you are looking for courses created through the Incubator, you can either find them all in the settings panel under the language you would like to learn (the drop down menu)
All of them are still beta, but you can learn them on desktops at the moment. If you select one, you should just be able to start the tree. Alternatively, you can see languages from your native language in the top right by the flag