i can't seem to download any of them
I used the FVD (Fast Video Download) add-on for Mozilla Firefox. It's free.
Open Mozilla Firefox.
Go to Tools > Add-Ons
In the top right you'll see a box for "Search all add-ons"
Type in "FVD"
Select Fast Video Download among the search results, and install it.
It will then prompt you to restart Mozilla Firefox - do so.
Now go YouTube / wherever and go to a page with a video you want to download.
In your address bar on the top right you'll now see a little blue FVD icon - click on it to select available video quality and start downloading the video.
There you go. :-)