Hi mikst
if download helper is installed correctly then you will have a little symbol greyed out(three balls) to the right of the browser address bar, when you go to youtube select the video you want to view as normal and the three balls should then become animated, to the right of the three balls there is a small black triangle pointing down, click on this and it should drop down a small menu with the name of the file you want to download typically it will have the extension of flv but not always, click on this and it should start downloading the video they are generally around 12MB for five or six minutes. once it has downloaded you can then open up the file manager under the work tab and navigate to the download directory where the file is stored, when you've found the file you can double click it and it will open in the media player.
and thats it! i hope thats enough.
cheers