Mike was right, but be sure that you use the 'air relief screw' on the top of the filter to vent out the air before adding the D.E. If you don't get ~all the air out first, the dry parts of the filter grid, or fingers, won't get the coating of DE they need.
As a side note, the DE 'slurry' is only needed when you only have 1 suction source, with more, the action of two sources removes this need.