Plugins Won't Activate.

Hi,

I'm pulling my hair out with this one.

I want to add new buttons to TinyMCE to do some custom inserts so
I've installed
http://deannaschneider.wordpress.com/2008/10/05/tinymce-custom-buttons-shortcode-example/

It's in the /wp-content/plugins/helloworld folder.

Logging in as admin this happens:
1) If you activate it, it won't stay activated. It says it's activated, but leave or refresh the page and it's deactivated.
2) If I activate it "Site Wide" then admin will have access to it, but others users won't see the new button, and won't even see it listed in the list of plugins available.

It's not only this plugin. Other plugins are also acting the same way.

Any ideas where to look?

Thanks,

...Mike