Because in university you are expected to take control of your own studies, and not to be spoon-fed everything. Also, you are not automatically enrolled in classes because many of them have different times that you can choose between. See
Class Search by Teaching Period
As it states in your course,
UNSW Handbook Program - Construction Mngt & Property - 3331,
your first year is fixed. Hence, to enrol for first semester, simply type in the codes given in Semester 1 on the handbook (i.e. BENV1181, BLDG1121, BLDG1211, BLDG160 for you) at myUNSW, and that will allow you to choose what times your classes will be.
You will also notice that from Year 2 you can choose electives from the pool of electives near the bottom of the page. But since your first year is fixed, you don't need to worry about that now (although you should be planning ahead).