The mat is the backmost part or background of a layout and your Style setting (under Templates menu --> Edit Styles)  specifies how the mat is to be shown when applied to a layout.

Updated November 2010

ProSelect includes the ability to use an image (which is placed in the Mat Backgrounds subfolder in your ProSelect Resources folder). The Image can be stretched to fit the shape of the layout (the left-most option) or aligned to a specific corner (useful if you have a logo in a corner of your background image) by selecting one of the Fit Image buttons.

And as of ProSelect 2010r1 and newer, you can also add  images to your Graphics Blocks (also setup under Templates menu --> Edit Styles).

ProSelect v3.x

In this older version of ProSelect,  you can't add a texture to a mat directly within ProSelect. However, some work arounds are:

1. If you are using the high-res production then output your composite as a layered Photoshop file, open it in Photoshop, apply the effect you want to the mat layer, flatten and save.
2. Create a template with a background image (Opening #1) covering the whole mat. Import your textured image into ProSelect and drop it into the background layer.