First get the Installer.app program on your iPhone.
Once installed, you should see the iApp-a-Day package listed in the Sources category under the "Install" section of the installer.
If not, then go to the "Sources" section of Installer (tap on the menu item at the bottom of the installer screen), then tap "Edit", tap "Add", and type in the address http://www.iappaday.com/install/ to add it.
The apps should then show up in Installer automatically as they become available.