Does natural selection still increase biological complexity?
There are multiple definitions of complexity, ranging from algorithmic complexity to computational complexity.  In biology, I'd say that algorithmic complexity (Kolmogorov complexity) is most relevant.  Unfortunately, we don't yet know enough about genetic circuits and all the other things that go on at the subcellular level even to make plausible estimates of an organism's complexity.

If the complexity of a human being were analyzed all the way from physiology down to the subcellular &amp; molecular level, I think it would be apparent that most of the complexity is present at the subcellular &amp; molecular level.  In fact, essentially all of the complexity of a multicellular organism is encoded at the subcellular level, so it's appropriate to say that's where the complexity resides.
It is not entirely clear that a human being, whose genome contains ~3 billion base pairs, is more complex than a protozoan Amoeba Proteus whose genome is ~34-43 billion base pairs. 

All of the processes that have been active in evolution since the beginning are still active, and changes in complexity - regardless of the definition of complexity - are still going on.  

The closest things there is to a "plateau" is a relatively static distribution of genotypes that can last only as long as the adaptive landscape is unchanged.  However, the adaptive landscape is always changing because of climate changes, plate tectonics, invasive species, disease, intra-species iteractions, and even because of chance fluctuations in the distribution of genotypes.