Attribute selection from dataset
You need to list the set of nominal values for the class in the arff file, comma-separated and enclosed in curly brackets {}. Here's an example:

@RELATION iris

@ATTRIBUTE sepallength  REAL
@ATTRIBUTE sepalwidth   REAL
@ATTRIBUTE petallength  REAL
@ATTRIBUTE petalwidth   REAL
@ATTRIBUTE class    {Iris-setosa,Iris-versicolor,Iris-virginica}


In your case, Iris-setosa,Iris-versicolor,Iris-virginica would be replaced with happy_pleased,...

If this still doesn't work, you can tell WEKA which attribute is the class in the explorer by choosing the correct attribute in the drop-down list above the histogram in the bottom right corner.