DE online or at your local pool store. I usually buy 25-pound box, $12-15...with a sand filter it'll last forever. There is a handle on the top of the black multiport valve, mounted on the top of the tan-colored filter. Like I said: IF the filter is "stuck" in recirculation mode....in that it's either in the wrong position or broken, by adding DE to the skimmer it should return to the pool like a white milky stream for a few seconds. pay close attention when you add DE....only 3-4 ounces in a slurry. On the multiport valve...A white ring outside the handle has words on it, like Waste/Recirculate/Backwash/Closed/Rinse. Please post a picture of the top of the multiport valve...looking straight down from above the handle. You have to move the handle to change the mode of the filter by pressing down on the end of the handle and rotating the multiport valve position. It locks in place in one of (usually) six spots. If it is not in the normal Filter mode, there is the problem.