From ModMyi - Wiki

Adding a repo to Cydia is extremely easy. First, you'll need to have a jailbroken iPhone with Cydia installed. Launch Cydia by tapping the icon.

NOTE: Installer sources are NOT the same as Cydia sources, and won't work. This is for Cydia APT sources.

In Cydia, tap the Manage tab at the bottom of the screen.

You will see a list of your currently installed repos (or repositories). In the top left of the screen, click Add.

Now, simply input the URL of the repo you are adding (e.g. http://apt9.yellowsn0w.com or whatever URL you are adding), and then tap Add Source.

Cydia will verify the URL, and then you should see your repo in the installed section.