It is really easy as[0]:

  git clone git://github.com/c9/core.git c9sdk
  cd c9sdk
  scripts/install-sdk.sh

to get c9 installed. But the license[1] :(

There is a cloud9 v2[2] alterantive but ide.c9.io just more polished.

[0]: https://github.com/c9/core#installation

[1]: https://github.com/c9/core/blob/master/LICENSE-COMMERCIAL-US...

[2]: https://github.com/exsilium/cloud9