Hit the Load button on the tab sheet Basic and select the learn data file. This file must match the learn data file format. The number of data tuples and variables is displayed, if the file is successfully loaded.
You have got to specify the output column, if it is not the first column in the data file. Enable the Classification task check box, if the problem at hand is a classification task, i.e. if the output is nominal. Finally you should check or edit the input variable types.