Since the shower drain smells there might be a couple of reasons behind it:
Shower’s drain p-trap (u-shaped pipe under the shower base) section has been improperly installed. Normally it should hold some water which creates a seal and prevents sewer gasses from escaping into the living quarters. However, an “s” trap might have been created instead (something similar to the top / right picture here http://en.wikipedia.org/wiki/Trap_(plumbing)).
Such installation might cause water to be siphoned out of the trap allowing sewer gasses into the bathroom. Unfortunately, if this is the case, repair would require accessing the plumbing pipes from underneath or removing the base. You can ask the plumber to use special camera to determine if this is actually the problem.
Second (very common) possibility is the toilet, unless you are absolutely sure that the smell is coming from the drain. Toilets have a wax seal underneath at the drain pipe connection. Those seals may occasionally require replacement due to leakage, toilets may not be fully secured to the floor flange which forms a gap between the seal and the toilet, etc.
You cannot see it, the toilet has to be lifted, but the result is usually the sewer odor and sometimes leakage from under the toilet or beneath it. If your toilet base / floor surface seam has been completely sealed with silicon (sometimes the rear section is left unsealed) you wouldn’t smell anything so it’s back to the trap under your shower base.
Let me know if this helped or if you have any other questions, just click "comment" below.