When I try to use the texture noise filter, it is greyed out. All of the other filters seem to be working fine. It doesn't seem to matter whether I have anything selected or not. What do I need to do to use it?
Check to see if the file is using the CMYK color space. Texture Noise is the only Eye Candy filter that does not work in CMYK, which would explain why it is greyed out. You could convert your image to RGB, run Texture Noise, and then convert it back to CMYK.
All other Eye Candy filters support CMYK directly.