When you select an organisational unit (or orgunit) from the organisation unit tree, datasets available for this orgunit will then appear as a drop-down list in the data set box as depicted below.

If the dataset field is disabled it means that there are no datasets assigned for the selected organization unit or that your user role is not authorized to enter data. For now we shall limit our concern to data entry. You will learn to create and assign datasets to specific organization unit in a later module.