As mentioned in the last image, we're going to extrude the polygon some indeterminate distance inward. It really doesn't matter how far, since it won't be affected by the corner above it. I suggest putting it as far as you want (which is why I didn't include numbers here).
Delete the face that would be blocking the toast slot. Ta da! You now have a toast slot that is almost perfect for smoothing.
But wait! Before you go to the next image, do you see what's wrong? It's the same issue we resolved by adding edges along the perimeters of our box. Where would you put the extra edges?
If you guessed "at either end of the toast slot, you're right! If we didn't do this, the edges of the toast slot would be far too rounded, and kind of ugly. Take a look higher up the modifier stack at your "Turbo Smooth" modifier to see what I mean. It's really unsightly, but we can fix that by selecting the affected area, connecting, and chamfering the selection.
Have a look at your whole toaster by clicking the "display end result" icon. It's just below the modifier stack and is shaped like a little cup. It shows you what the toaster will look like after all the modifiers are in place, while still allowing you to work within the modifier stack.
If you can't find it, do an F1 search; or just deselect your toaster entirely, and 3DSMax will show you the whole object.