To download a single file from the FTP, first log in to http://data.itelescope.net/
Once there, locate the file you wish to download by first double clicking the telescope folder (this will match the telescope you took the images on), t13 in this example.
Then, double click the target of the images that you entered, and you will see a series (or one, dependent upon use) of folders that are labeled by date.
Next, located and double click the folder that coincides with the date the images were taken (in UTC), this will take you to a page that shows a variety of files to download.
Finally, double click the file you wish to download and your browser will download it automatically.
When it has finished downloading locate the zip file in (usually in your downloads folder on your computer) and extract the file to your chosen location using a program like winrar, winzip, or if using windows 8.1 and above, the windows auto-extractor.