How do I divide a dataset into training and test set?

You can use the RemovePercentage filter (package weka.filters.unsupervised.instance).

In the Explorer just do the following:
  • training set:
    • Load the full dataset
    • select the RemovePercentage filter in the preprocess panel
    • set the correct percentage for the split
    • apply the filter
    • save the generated data as a new file
  • test set:
    • Load the full dataset (or just use undo to revert the changes to the dataset)
    • select the RemovePercentage filter if not yet selected
    • set the invertSelection property to true
    • apply the filter
    • save the generated data as new file