1. Open the form of that step whose field you want to make mandatory.
2. Go in Details window, click Application Tab and choose Entry Pageflow.
3. Click on Add button, give form name for that step and click Ok.
4. Now select that name of form and Click Edit.
You will get enter into Bonita Form builder.
5. Click on the field you want to make it mandatory.
6. Choose Options tab and tick Mandatory check box.
7. Press Ctrl+s to save and its done.