I'm not near my iPhone, otherwise I would have tried it myself before posting this. I'll definitely be trying it later. I'm a big Zork fan and it would be fun to geek with. My son is 11 and I've been watching him go through Zork I and offering a few hints along the way.
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.