You obviously are doing it wrong.
Copy the web page address of the youtube video you want and paste it in the obvious point on the page given and click on the download button to the right of where you entered it. Wait for the page to reload with the list of the formats you can download the videos in. Click on the one you want and save it to your computer at the location of your choosing. You should then have that file saved on your local computer and be able to play it at will without any Internet connectivity at all. It is a file, copy/move it as you see fit.
Done.
If you installing anything, getting asked for your credit card, etc. - you did it wrong.