Raddish,
Since you have a 2 speed pump, given a choice, I'd try adding the DE while on low speed. You want it to form an even layer on the top part of the sand and it seems to take a while to layer the sand. The 15-20 minutes mentioned. Also, haven't seen it mentioned so far in this thread, but you should pre-mix the DE in a few gallons of water and pour it in the skimmer slowly. I even think they say this on the DE package.
Al