You have two ways. On Installer there is one app you can download which emulates the original three Zork episodes.
I use the second way, which is to install iFrotz (also available on Installer). This is basically just a z-code app, so you can run zork as well as most all other Infocoms and just as good the hundreds of z-code games puzzles and "books" that have been written since then.
If this makes no sense to you ask. 😉