If you have a counter with particle board, wood trim or some material that does not handle moisture well, then your dishwasher can damage the underside of it. The steam that escapes the appliance when you open it after a cleaning, or that escapes from it if the seal isn't working, can make the wood of your counter swell and warp. It is best to install the cover plate that comes with the washer to deflect the moisture in that location.
Take the plastic cover off of the plate that comes with the dishwasher. Try not to scratch the metal, since this might cause it to rust.
Squeeze a tube of silicone sealant along the edges of the cover plate. You should have a ridge line to follow, but if not, simply apply it around the edge. If no sealant comes with the cover, you can pick it up at a hardware store or use a caulk.
Place the counter cover up under the counter and centre it. Set it up next to the edge of the counter. If there is a seam between materials under the counter, the plate needs to cover it.
Hold the plate, or cover, in place and tap the nails in that came with it. Leave the cover alone to allow the silicone to dry for an hour or two before using the dishwasher.
If you have no cover plate with your dishwasher, even brushing on a wood sealant will help. Brush it on under the counter surface up to the edge of the counter all the way across the front of the dishwasher.