Screw fell into drain through one of the little slots. How can I get it out? Will it cause issues if there?

You better get it out before running the dishwasher. Slide it out and look under it where the hoses are located and use a magnet to get the screw. You mat even be able to remove the cover where it fell into the drain and use a magnet to retrieve it. Unless you get it it will cause a lot of damage to your dishwasher. Always plug up holes where a screw can fall with a rag before doing any work removing screws. Here are some manuals that may help: https://dl.dropboxusercontent.com/u/8010163/Whirlpool_Dishwasher_Service_manual.pdf
https://dl.dropboxusercontent.com/u/8010163/whirlpool_dishwasher.pdf
https://dl.dropboxusercontent.com/u/8010163/Whirlpool_Kenmore%20Dishwashers.zip