Need to fill nail holes in fiber cement siding

The moisture barrier would generally go between the furring strips and the concrete and would be attached to the wall simply by attaching the furring strips to the concrete. If you wanted to really seal the area you could run a bead of adhesive sealant between the moisture barrier and the wood.

6mil plastic is a "vapor" barrier, not a "moisture" barrier. The difference is that a moisture barrier allows air to permeate it but prevents liquid water from going through. A vapor barrier prevents water AND air being transmitted through it. Moisture barrier is used between porous materials like concrete and wood to allow the various materials to acclimate to changing temperature and humidity without exchanging liquid water (the concrete will absorb a lot of liquid water from the ground, and then pass it into the wood which is a sponge for water, which will cause the wood to rot). If vapor barrier is used, moisture will be trapped on either side of the barrier, which can cause puddles of condensation on the moister and/or warmer side of the barrier.

So, to answer your question, you should probably use a product like Tyvek or TyPar, or an older material like roofing felt, between the furring strips and the concrete. For an installation like this, you can just cut strips of the barrier to prevent direct contact between the materials, and then simply place them behind the furring strips as you attach the strips to the wall.

"If you have some small cracks and holes in your siding, you can fill them in with patching material specifically designed for this purpose, available at your local hardware store. Keep in mind that the patching will be obvious, whether or not you paint over it, so this is the best solution if the cracks and holes are in out of the way places that aren’t readily noticeable. If you have really big holes and cracks, then the siding will probably have to be replaced altogether."

Source: http://www.doityourself.com/stry/how-to-repair-fiber-cement-siding#b