Install installer package on an installed system?

Try:

sudo apt-get install ubiquity-frontend-gtk

If you're looking to tinker with the installer, please do so within the confines of a running live CD, preferably using a virtual machine like KVM. With some very small exceptions, ubiquity is written in Python, so you can install your favorite editor and modify it in place.

Equally, if you're aiming to make large changes, you can get the latest copy from revision control by running bzr branch lp:ubiquity. You can build a set of Debian packages using debuild, then copy them to and install them in the live environment. Alternatively, you can NFS mount the portion of the ubiquity tree that you're working on in the live environment over top of the existing directory, edit files in the branch on your local machine, then immediately test them by running the installer.

The latter approach is a stripped down version of what I use while working on it, and I find it speeds up my development, prevents me from losing changes if the virtual machine goes down, and prevents my changes from becoming out of sync with changes to the branch.

Do you mean Ubiquity? https://wiki.ubuntu.com/Ubiquity