Open the texture image. Create a seamless tile from the texture. (I use a filter for this. Simple Filters puts out a good free filter for making seamless tiles but there are many other filters that create seamless tiles.)

Once you have a seamless image selected – Edit – Define Pattern.
The image will now be in your patterns folder, and available as both a pattern overlay and a texture overlay.

To apply it as a texture – open your layers palette – go to the bevel and emboss section and in the sub category – texture –you can browse to your new texture in your patterns folder.

You can adjust the depth and scale of the texture and of the bevel to get the right texture for your image.