The recipe instructions are either unclear or uninformed. Adding the corn meal to the end of the boil as the instructions imply is not correct technique. As Yooper and your LHBS said corn meal needs to be mashed. It also needs to be gelantinized prior to mashing. If you want some corn flavor to an extract beer your best bet would be to do a mini mash with pound each of flaked maize and pale malt. I don't know where you found that recipe but to be kind there are better ones around.