So you can't even see the top Octane menu bar, or add the Octane shelf, right? Weird, the HOUDINI_PATH variable seems right to me, but to be sure, can you please attach your houdini.env file to test it here?
Are you sure that you are editing the right houdini.env file? Be aware that each Houdini main version (for example H16.5) has its own $HOME folder with the file inside, and also be aware that each user has a copy of this folder, so perhaps you are editing the houdini.env file of another user.
Computer Graphics Software Developer
Octane plugin development and support: LightWave and Houdini