From ModMyi - Wiki

This is a guide on how to add rogue sources to Installer for your iPhone or iPod Touch.
Adding these sources will give you a greater array of apps and themes to customize your device.

Step 1

• Open up Installer.app. Make sure you have the latest version of 3.0b8
• Select the Sources tab
• Click the "Edit" button

Step 2

• Click the "Add" button

Step 3

• Type in the full URL of the source you want to add, including the "http://" For instance, if you're installing the official ModMyI.com source, it is http://modmyi.com/installer.xml (careful on the case sensitivity there, the iPhone likes to capitalize .XML, but it shouldn't be capitalized).
  
• Click OK

Step 4

• Wait while Installer loads the new source.

Step 5

• Click the "Done" button

Step 6

• Close Installer by pressing the Home button.
• Open Installer again and the source you added will now have a name and you can access it's files.

NOTE: Images by CyberGreg and Poeticfolly.