I could not tell from the article, but is this the type of mutation that's present at birth?

Yes, that is correct. Most neurons are post mitotic i.e. the cells do not divide in adult life so the mutation would need to be present early in embryonic development.

How does the brain grow if no (or few) new neurons are created post-birth?

Strictly speaking, there is still some cell division shortly after birth and into the early phases of post-natal development. So the brain increases in size, slightly, via that method. But mostly it is from the elaboration of the dendritic arbors of the existing neurons. The size and branching factors increase many times during development. Check out the beautiful (and stunningly accurate) drawings of the famous neuro-anatomist Santiago Ramon y Cajal.

For example, the Purkinje neurons of the cerebellum:

http://www.gladstone.ucsf.edu/wp/2009/06/knowledgegrows/

Note how the number of branches increases to sample more of the volume.

More good new from Northwestern hopefully it leads to some form of treatment. My best friend's dad died from ALS a few years ago, it's an absolutely horrible disease--finding a treatment would would be spectacular.

I would like to know of any diseases which are successfully treated by the techniques described above. Have there ever been any certifiably broken biological pathways, indicated through genetics, that have been corrected through drug therapy? It seems like most drugs have very broad effects and don't adjust an actually diseased pathway. Even monoclonal antibodies for inflammatory diseases seem to be a poor match for the specific treated diseases.

A large fraction of modern drug development is focused on specific biochemical pathways, often identified via the study of rare genetic disorders or genome-wide association studies. For example, patients with Familial hypercholesterolemia, a relatively rare genetic disorder, provided strong evidence for a causal role of cholesterol in ischaemic heart disease (a discovery for which Brown and Goldstein were awarded the Nobel prize). The enzyme inhibited by statins (HMG-CoA-reductase) is a key player in this pathway.