Shipping module causing errors on site

Hi, I recently installed this shipping module to calculate Australian postage. However I've noticed that since I installed it, there have been some problems. When I click on "estimate shipping", it comes up with a blank page. The only thing on it is a link that says close window. Also, when I go to "checkout" or "checkout with paypal", it sends me to a "500 internal server error" page. Is there anything wrong with the files in this module that could cause this, or something I may have done wrong when installing it? I am using ZenCart version 1.3.9d.

I would like to try to make this one work before I install any other AusPost shipping modules. Also I am not hugely experienced with ZenCart or zc terms so please explain and simplify everything in your answers..!
Thank you!!