Where are my Audio Units? The only ones I can see are Apple ones.

Have you just upgraded to GarageBand 10.0.2?

I am seeing the same - if I open an existing project, the "Audio Units" option is dimmed and cannot be changed. But for a new project it is active, right after creating a new project.

Try to create a new project and see, if you can enable the audio units for new projects.

Are all of your Audio Units compatible with GarageBand 10.0?

See: GarageBand for Mac (v10.0): Unexpected behavior with Audio Units plug-ins

The new version is not at all intuitive to use.

:-(

It is more like Logic than GarageBand. I am using the easy way out and set up my projects in GarageBand '11 and then open them in GarageBand 10.0 to add drummers and arpeggiators. And sometimes finish the project in Logic, if I need to edit the pitch of vocals.

You may want to have a look at the Bullets&Bones FAQ. There is a new minute video tutorial:

http://www.bulletsandbones.com/GB/GBFAQ.html#gbv10audiounit

This FAQ entry offers a Minute GarageBand video tutorial

(Let the page FULLY load. The link to your answer is at the top of your screen)