Taken from: The WordPress MU Manual
Mu-plugins are the plugins located in the “mu-plugins” folder; these are activated by default affecting all blogs on the WPMU installation.
All mu-plugins downloadable from Premium plugins have been developed to be 2.8.1 compatible. This is the largest collection of mu-plugins in one place.
Installing mu-plugins
1. Download the mu-plugin
2. Unzip the file to extract all files into a folder on your hard drive
3. Check the zip file for a readme file or install file
4. Make sure you read these instructions in the readme file or install file carefully – - many mu-plugins require additional steps to install other than those explained below! If you don’t follow these instructions correctly the plugin won’t work and may cause problems on your site!
5. Launch FileZilla
6. Enter your domain name, your username and password. NOTE: the same username and password that you use to log into your cPanel.
7. Click Quickconnect
8. Locate and open up the mu-plugin folder (so its php file is visible) that you want to install on your computer.
9. Open up your mu-plugins folder for your site (located within wp-content folder of your public_html folder)
10. Select the php file inside the mu-plugin folder on your computer that you want to install and drag it into your mu-plugin folder on your site.
11. Complete any additional steps as specified in the readme file or install file!
12. Once you have completed all these steps correctly the mu-plugin should automatically be activated on your site.
Next up, we’ll be covering running SQL code on your database!
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July 13, 2009 
