Open up your transparency palette, if it’s not already open click Window->Transparency. Click on the small arrow in the top right of the palette and choose “Make Opacity Mask” from the list of options that appear. Then click File->Place and drop in your texture that you are using. Resize it and adjust it until you have the texture you like. Make sure to click back on the image thumbnail within the transparency palette when you are done to get out of the transparency mask mode that you are in.