PACE units link your study with the 'real world' and most involve internships. Every Macquarie undergraduate degree requires you to complete at least one. Some have research projects, fieldwork, stimulations, etc.
So a PACE unit may involve you doing a research science project at a local school, working on a project for a company, etc.
I suggest you email the PACE faculty who can directly answer your questions.
On a side note, PACE may not seem like alot of fun but you can put it on your resume at the end. Go in with an open mind, and be prepared to apply what you've learnt in your degree to it!