You may have to get DE at a pool store. I'm not sure if places like Lowe's or Home Depot sell it in their pool supply section or not. It is added in the skimmer. Buy the smallest container of it that you can find which may be around 10 lbs. But, I wouldn't add the DE just yet. Since your water is still green, you've still got algae to kill and will most likely be backwashing frequently and will just end up throwing the DE out with the backwash. Kill the algae and then if your filter is having trouble filtering out everything, then that is the time to try adding a little DE.