Actually, evolutionists have a few explanations. See, for example, the chapter 'From Single Cells to Multicellular Organisms' in Molecular Biology of the Cell, one of the leading cell biology textbooks. John Tyler Bonner, a developmental-evolutionary biologist at Princeton University, has written a whole book on the evolution of multicellular life.
There are three main theories for the evolution of 'multicellularity':
The colonial theory: this holds that the symbiosis (cooperation) of many organisms of the same species led to a multicellular organism. This theory has the most support in the scientific community and has the advantage of aspects of it being observable. Some species of algae form colonies of thousands of cells, only a fraction of which reproduce. Cooperation between cells and formation of groups of cells has also been observed in bacteria.
The symbiotic theory: this suggests the first multicellular organisms resulted from symbiosis of different species of single-cell organisms, each with different roles. Over time these organisms became so dependent on each other they could not survive independently and were incorporated into one multicellular organism. The problem with this theory is it is not clear how each organism's DNA could be incorporated into a single genome.
The cellularisation theory: this states that a single unicellular organism, with multiple nuclei, could have developed internal membrane partitions around each of its nuclei. The problem with this theory is that while there are some existing single-cell organisms with multiple nuclei, these nuclei are either the result of the fusion of uninuclear cells or each nucleus has a specific function (making it unlikely the nucleus could split from within the single cell).
Since the publication of The Greatest Hoax on Earth there has been some interesting research on the evolution of multicellular organisms. William Ratcliff and his colleagues at the University of Minnesota conducted an experiment in which they evolved multicellularity in unicellular yeast. The unicellular yeast in their experiment formed multi-celled organisms that stayed multi-cellular and reproduced as a multicellular organism.
Critics of Ratcliff's experiment have pointed out that many yeast strains naturally form colonies, and because yeast has ancestors that were multicellular, the experiment might merely have activated a vestigial gene already present in the yeast. In other words, Ratcliff's experiment may only have demonstrated the reversion of a unicellular organism to multicellularity, rather than the evolution of multicellularity. This latter point was also picked up by creationists, as it supports their claim that evolution cannot produce 'new' genetic information.
Ratcliff maintains the traits that evolved in the yeast in his experiment were novel and they improved the fitness of whole clusters, not their component cells, which demonstrates evolution in a multicellular manner. Furthermore, much of evolution proceeds by using existing traits in new ways, which is what Ratcliff's yeast did. Ratcliff and his team now intend to repeat the experiment with algae (that has no multicellular past) and they are continuing with the yeast strains to see if they evolve further into true multicellular organisms.
So Sarfati's actual point is that evolutionists don't yet understand exactly how single-celled organisms evolved into multicellular ones. Much of the rest of chapter five (Sarfati's discussion of the programming and organisation of information) is really a variation of the argument about the origin of life, which is the focus of chapter 13 of The Greatest Hoax on Earth
 Bruce Alberts, et al., 'From Single Cells to Multicellular Organisms', in Molecular Biology of the Cell (third edition), New York: Garland Science (1994); available online at http://www.ncbi.nlm.nih.gov/
 John Tyler Bonner, First Signals: The Evolution of Multicellular Development, Princeton: Princeton University Press (2001).
 My summary of the theories is adapted from Wikipedia: http://en.wikipedia.org/wiki/
 Gregory Velicer and Yuen-tsu Yu, 'Evolution of novel cooperative swarming in the bacterium Myxococcus Xanthus', Nature, Vol 425 (2003), pp 75-8; Paul Rainey and Katrina Rainey, 'Evolution of cooperation and conflict in experimental bacterial populations', Nature, Vol 425 (2003), pp 72-74 http://www.icts.res.in/media/
 Lynn Margulis, Symbiotic Planet: A New Look at Evolution, New York: Basic Books (1998), p 160.
 C Hickman and F Hickman, Integrated Principles of Zoology (fifth edition), Mosby (1974), p 112.
 William Ratcliff, et al., 'Experimental evolution of multicellularity', Proceedings of the National Academy of Sciences (17 January 2012) http://www.pnas.org/content/
 Bob Holmes, 'Lab yeast make evolutionary leap to multicellularity', New Scientist, Issue 2818 (23 June 2011) http://www.newscientist.com/
 Elizabeth Mitchell, 'News to Note' (2 July 2011) http://www.answersingenesis.
 Carl Zimmer, 'Evolving Bodies: A Storify Follow-up' (18 January 2012) http://blogs.discovermagazine.