Yes, I believe it is the MessageDialog. Here's a version that should work, provided you have the latest version of CDF player:

Manipulate[
 Column[{Labeled[
    NumberLinePlot[{n}, PlotRange -> {{-6, 6}, {-1, 1}}, 
     Ticks -> {{0}}], "x", Right], 
   Style[If[c, "Correcto", "Incorrecto", ""], "Subtitle"]}], {{n, -4, 
   ""}, {-4, 3, 0, -2, 5}, Slider}, {{c, Null, ""}, 
  Row[{Button["Positivo", c = Positive[n]], 
     Button["Negativo", c = Negative[n]], 
     Button["cero", c = n == 0]}] &}]

Your original example depends on a user evaluating the code in the notebook, which they cannot do in CDF Player. So to remove this requirement, you would need/want to use DynamicModule. And note the use of SaveDefinitions in the attached.

http://reference.wolfram.com/language/ref/DynamicModule.html

Ian

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