plugin does not show up to activate

Just reiterating these instructions:
1. Upload ‘registered-only.php’ to your plugins folder, usually wp-content/plugins/
2. Verify that the “Anyone can register” option in the Options->General page of the WordPress administration interface is NOT selected
3. Activate the plugin on the plugin screen

The plugin seems to be 8 months old. If you use WP later than version 1.5, have you checked with the plugin author that it is compatible with your version of WordPress?

i am using 1.5 and followed those steps and had no luck.

Well it should work for 1.5, according to the author and some of his users. I believe you have seen the discussion under this post at the authors blog: https://carthik.net/blog/vault/2005/05/12/registered-only-plugin-reworked/