First, create a New Channel in the Channel Palette and choose the Channel Layer.

Next, choose the Rectangular Marquee tool from the tool palette and create a bunch of white boxes in the new Channel layer you just made. Something like what's shown at left.

Try to avoid very thin boxes as these will round out probably more than you want on their ends.

Now, you'll want to go to the Filter menu, and choose Blur/Gaussian Blur.

Blur the boxes channel until the corners look rounded and blurry, like the one at left.

Don't worry about what your Blur settings should be, eyeball it so it looks like the image at left.

Next, choose Levels (Image/Adjustments).

Again, numbers aren't important here, just make sure all 3 triangles meet towards the middle (but don't overlap them). Before you close the Levels window, look at your image.

If your edges look TOO crisp and jagged, you moved the triangles too close. If they still look blurry, you need to bring them closer together.

Here's our previously blurry boxes with nice sharp rounded corners, ready to turn into a selection to create our interface plate.

So the next step is to create the dirty, pitted metal look by using this channel to create a selection and filling it with clouds.

Full details on how to do that are in my other tutorial-Dirty, Pitted Metal to finish off your rounded corner interface plate.