Normally, you can choose to change a filter into a radio or checkbox filter by right-clicking on the filter.

However, sometimes you will right-click on the filter and be unable to see the option to change to Radio or Checkbox.  This occurs when your imported design table column was set to numeric or character, instead of factor.  This would occur most often for columns like Time, where the user may have Time 1, 2, 4, 8, etc.., or in cases where there were a large number of different groups.

The column type can be changed, by going to the Table Menu | Columns | Column Properties for your design table for the data set in questions (or for a previously generated Table object).  A shortcut to getting to this screen would be to right-click on the column header of your design table or table object.

Then, change the Column Type setting to Factor. This will update your filter and allow right-clicking to change to a checkbox or radio filter.