Washers, or some other sort of shim, are the way to go here. Weight will accomplish precisely nothing. Depending on whether the plank was properly dried before use, there's a chance it will continue warping; shims will allow you to adjust the leg lengths again once the bench has stabilized.

You need to run the bottom surface of the slab across a jointer to flatten it. Or, hand plane flats where the legs attach. Of chisel mortises into the wood where the "high" legs attach, so they sit level. Or get legs with leveling screws built in.
Casey

I don't know where you live, but in most cities they have businesses that provide sanding service. They have giant belt sanders that they can run your project through and make it flat and level on one side or both sides. You need to tell them you want maximum thickness. If you do this, be careful you don't over do it and have it sanded too much. You could also mortise in a slice of a red oak branch to use as a shim.
Are you sure your piece of wood is dry? I hope so.

Thanks everyone for your suggestions. The wood is dry so we do not anticipate further warping. We have elected to use a shim to fix the problem. What a help this forum has been. :)