I don't believe the precise the genetic basis for tail growth is known, but I'd say the best bet is Hox (homeobox) genes. Mutations in these genes cause alterations in the development of the axial skeleton (vertebral column and ribs) and limbs, among other things. They are conserved across all animal phyla, with similar or homologous functions. To give you a tail, I'd have to know the exact pattern of expression of the correct gene(s) and how to achieve it by artificially engineering promoter-gene fusions and inserting them into the genome of an embryo.

There are a few problems with this. First, very little is known about the exact control mechanisms of Hox genes, or how they interact synergistically to produce the animal's body. I'd have to do a lot of experiments on mice and/or apes, and there'd be a lot of nasty messes produced - horrid things with deformities too hideous to describe, which would die in utero if they were lucky. Then I'd have to try it with human embryos.

Secondly, you may notice I said embryos. Unless your body works a lot differently from those of most mammals, causing it to grow a tail in adulthood would require a lot more genetic engineering, probably using tailored retroviruses or tissue engineering. Making it functional is a whole other problem - your brain is not really developed to control a tail. Do I have to alter it too?

In conclusion, this molecular biologist says: "Go the cybernetic route, it's a lot easier!"