How to make a circular selection of a photo

Is the plate a perfect circle in the photo (or is it a distorted eliptical shape)?  To create a perfect circle, use the Shift key and click and drag the elipsis tool over the plate.  You can also use the Alt key after you click the center of the circle/plate and drag a perfect circle out from the center.  For more information about selections and each of the selection tools, see this portion of the User Guide.

You haven't told us which version of Elements you are using.  If you are using a recent version, you may find that there are other tools available that will make creating a selection of the plate easier.  For example, in the following screenshot, I used the Select Subject button to select a plate and it worked fairly well:

You may find that the Auto Selection tool will do what you want by simply dragging a square marquis around the plate.  Frankly, the eliptical marquis is one of the more difficult tools to make a selection.

If you need more help, I suggest you post the photo you are working on and we may have other suggestions.  There are usually multiple ways to perform a task in Elements.

Make the selection.

Select > Transform Selection and drag the selection as needed and press the Enter key or click the green arrow.

Select > Inverse and press the Delete key.