In order to define least squares effects, you must:
1. On the Task Roles page, assign at least one variable to the Classification role
2. On the Model page, add the classification variable as an Effect.
3. On the Post Hoc Tests->Least Squares page, click Add -- it will add the classification variable as the selected effect. If you have more than one classification variable assigned as an effect, then you can select among them.
Hint: the task window does show some helpful messages in the status bar, providing cues as to what is required to use the controls on this page.
Chris