If the stat tool has not automatically identified your column correctly, you can explicitly change a column's type in two ways.
Click on the Column Type Cell
As you click on this cell, it will cycle through all the available column types. The frequency column type is not always available.
Right Click Anywhere in the Column
Column types (auto, data, label, group, frequency and formula) can be set using the data view's right-click menu.
Frequency Columns
The frequency column type will only be available if the column is to the immediate right of a data or group column (or another frequency column). Frequency columns always "belong" to a group or data column to their left. If this is not possible, the frequency column option will not appear.