Below 1 psi, gas pressures are measured in inches water column (WC). Low pressure propane is 11" WC (as opposed to high pressure, which is usually in the 10-20 PSI range). Since you bought low pressure burners, you need a propane regulator that outputs at that pressure. Brewer's Hardware sells one
here. I've also seen posts recommending a Marshall 290 or 299.
Alternately, if you'd rather go for high pressure but don't want to return the burners, it might be possible to swap out the oriface -- I don't know where you'd get one for that burner though...