Your datafile should be a .csv file or .txt file. If it is formatted as a .xls or .calc, open it with your usual software for data management and save as .csv file prior to loading it in this application.
Uncheck 'Header' if the first line of your file does not consist in column names. Select the column separator of your datafile. By default, for a csv, it should be ',' (English) or ';' (French). Select the decimal separator of your datafile. By default, it should be ',' (English) or ';' (French).