WordPress.org

I've successfully installed a plugin/theme but it doesn't show up on the plugins/themes screen.

If this happens you can't activate the plugin/theme because the content of the ZIP file that contains the plugin/theme is broken. There are some plugins/themes around that have some extra directories in their ZIP files, e.g.:

• "wp-content/plugins/[plugin-name]/[files]"
• "wp-content/themes/[theme-name]/[files]"
• "[plugin-name]/[plugin-name]/[files]"

I don't really want to support this stuff since this would lead to some extra guess-work.

I recommend this: if you encounter a "broken" plugin/theme just contact the author and politely ask him whether he might fix this. The standard layout of a ZIP file should be "[plugin/theme-name]/[files...]".