Did you attempt to create the Mat in the material room first?  If you can provide the steps or confirm that you used the steps in the attachment markschum posted, that would help in solving your problem.

Edit:  I just reread your post, if you followed the directions to the previous post, the MAT Pose file should appear in your pose section of your library.  Load your clothing into Poser and have it selected.  Double click the MAT Pose icon in the library.  That should change the texture of your clothing to the new texture in your library.