How can one tell from the MO diagram of the cyanide ion that the HOMO is carbon-centred? The cyanide ion, when acting as a nucleophile, typically attacks via carbon as its HOMO has a greater contribution from carbon. But how can we deduce this from the MO diagram? 

To me it appears that the nitrogen $\mathrm{2p}$ orbitals are energetically closest to the $3\sigma$ HOMO, so its coefficient should be larger in the HOMO. What is wrong with this line of reasoning?

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The 2s and 2pz atomic orbitals are of the same symmetry in the molecule. That means that the 2 sigma* and 3 sigma are actually hybrids. The former is a hybrid were most of the electron density points to the bond (making it partially bonding), while in the latter most of the electron density points outside, i.e. a lone pair in the carbon.