In order to ask a question related to a specific unit go to the specific unit and under "References and attachments" you will find already asked questions from other students or you will be the first one asking. Click therefore on the green "Ask new question" button and fill out the short form; choose the category of the question and the unit. Be as specific as possible, so that other students or the instructors can easily help you.
You can also ask a unit related question inside the Discussion Forum. Here the same; fill out the form, choose the category and at the end the unit.