Rose_Punk, you only really need to go on the compulsory day because you will get information specific to your course. The other days just have seminars on note taking, referencing, reading and other skills like that...you could even go to some of the seminars on your compulsory day if you wanted. I went to part of a referencing one to get the sheets, then left - I heard they weren't very helpful because they almost state the obvious so unless you aren't confident with some of the skills, then maybe you should go.
rego2, you probably have one compulsory day which you should probably attend. I don't know what happens if you don't, and there are probably a few students who don't go anyway. I went to mine today and I didn't get my name marked off or anything so there is no record of me attending. You can get a student card if you go to Student Services (I think that's where it is...either way there are heaps of signs advertising where to go for them), and I'm sure you can do that any time, not just during Orientation.