Using a PC:
To unzip a file, right-click and choose "Extract All.."
Windows will then launch a wizard that will allow you to choose the location where you would like to extract the ZIP file.
Using a Mac:
If you are using Apple Safari, your files will download to the location you have set in Safari >> Preferences as the default download location. If you have not selected any particular destination for your downloads, your files will download to Users >> [Your Name] >> Downloads. An instance of this window will open to show your downloads.
If you have the "Open 'safe' files after downloading" option selected in Safari >> Preferences (a default setting), your files will automatically unzip when they are downloaded. If you have deselected this option, your files will appear in ZIP form. To unzip these files, simply open them by double-clicking.