You can't use MAT poses on a prop. Only in the case where it's parented to a character. A character must be in the scene for a MAT pose to work. So that's why it won't work. You could, if you have Pro Pack or P5, take it into the Setup Room and make it a character (a character with only 1 body part, but still a character). The little plus sign will appear when something is loaded that you can use the things in the palette for, again that's why it won't appear as long as you have only a prop loaded.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.