Well, it's been a few years since I've been in Chem, but basically, polar molecules come about because the bonds between atoms in a molecule don't have a symmetrical distribution.
So for water, the 2 hydrogen atoms aren't spread 180 degrees apart (which would be symmetric and "balance" the charge distribution of the molecule), but rather are something like 120 degrees apart (that number is straight out of my butt, so don't quote me on it, but it is somewhere between 90 and 180). As the atoms in the molecule have a charge to them (O +ve, H -ve, I believe) there is a polarity to the molecule.
Most hydrocarbons (but not all by any means) have a symmetrical distribution of mainly hydrogen and carbon. A chain of carbons (e.g. 5 for pentane, 8 for octane) with hydrogen atoms lined up 2 by 2 running down the chain like legs on a centipede, with an extra at each end of the chain. Since any charge partitioning between the carbon and hydrogen is balanced by symmetry across the molecule, there is no net polarity to the molecule.
Then there is the fact that they say they are using magnets to ionize the fuel. Well Ionization implies imparting a charge on the molecules by adding or removing electrons. Since your fuel is neither a ferrous metal, nor a conductor that will pass a flow of electrons, the magnets should have negligible effect on the fuel. Further, if the fuel passes through a steel fuel line within the active field of the magnets, the magnetic field will be channeled by the fuel line, and greatly weakened within it (the reason magnets on the oil filter or outside of the oil pan don't help trap iron particles).
blah, blah, blah. I'm sure there are one or two technicalities I am wrong or unclear on, but generally, putting magnets on your fuel line will do dick-all, and a hydrogen generator will net no gains in economy from combustion of the hydrogen (I could be convinced though that it might net a few percent increase in economy if it helps the gasoline burn more completely.... but that would require a good explanation and testing from an independent lab). If you want to save fuel, look at proper tuning, gearing, and maybe something to help the aerodynamics at the back of the truck (this I have thought about many times, but haven't done any experimenting yet).
Hope this helps a bit. And to anyone with a better chem/physics knowledge, flame away.
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