You must be in the Material Room for the + sigh to appear. You can apply mats from everywhere but only add new ones from the Material Room itself.

So be sure you're in the Material Room tab :)

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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.