To load a model:

  • In the explorer, right click in the result-list area and then choose the ``Load Model'' option.
  • When using the command line, there is a Weka option -lLoads classifier from the given file. You just need to give this as argument to the weka.classifiers.Evaluation class.

    For example:

    java weka.classifiers.Evaluation -l treeclassifier.model 
       -d trainingset.arff

Source: Weka FAQ