A paper presentation is oral (10 minutes in length, with 3 minutes after that for questions). A poster presentation is your work on a poster (for MUPC, this size should be 42 X 42); for this presentation, you will stand by your poster and answer questions from those who come by to look at your work across a one-hour session.
The basic standard for MUPC is business casual.
In general, the expectation is that you should attend the entire conference, attending presentations and viewing the posters you find interesting. (You can see the Schedule in the program tab above under "More". A more detailed schedule will be available via a QR code at the conference.) At the very minimum, you need to present for the entire poster session or presentation session in which you are scheduled.