This page explains ezNZB’s “Step 4 – Import Your NZB File” screen. You can ask a question down at the bottom of the page.
This should be the easiest step, but just because you have downloaded an NZB file doesn’t mean that you will be able to find it on your computer. For some reason, many web browsers don’t allow you to select a location when downloading a file. Instead, the download folder is specified in the browser’s settings and all downloads automatically go there.
So, if you downloaded an NZB file in Step 3, but can’t find it, poke around in your browser’s settings and look for something like: “Save files to” and you will see the folder. You may want to change the setting to the desktop “folder” so that you don’t have to dig through folders to find the files that you just downloaded.