can anyone show me how to use the gog downloader please

I'm assuming you mean the original GOG Downloader, not GOG Galaxy.

First you need to download and install it:
https://www.gog.com/downloader

Then, while visiting GOG using a browser, you
1. open your game library using the navigation bar at the top, Account -> Library
2. click on the game you want to download
3. click the button that says "More", then "Downloader Links"
4. click on the file you want to download(grey file name), the GOG Downloader should now open and start downloading

Hope that helps. Welcome to GOG.

Midoryu

Edit: I was using two browser extensions, tried without them. Description altered.

Also, don't forget to click on settings on the GOG downloader, and choose the destination folder ('Save downloads to'). ^_^

If you really meant the legacy GOG Downloader program, the instructions others have provided above should do the trick however keep in mind that that program is no longer officially supported by GOG and will be discontinued at some unspecified point in time in the future.

If you're just looking for any method to download your games however, the recommended way is to use GOG Galaxy client beta currently which you can obtain from https://www.gog.com/galaxy If you encounter any problems with it, it is good to report bugs to their bug tracker at http://mantis.gog.com or to contact customer support at the link at the bottom of the page.

Alternatively if you're not fond of Galaxy and do not wish to beta test, in addition to the old unsupported GOG Downloader you can directly download games via the web browser, or use a browser download manager program as well which many people use. I don't use those so I can't recommend any.

If you do decide to try out Galaxy client to help beta test it, you may also be interested in the GOG GALAXY BETA thread here in the forums where you can provide feedback: https://www.gog.com/forum/general/gog_galaxy_beta_2

Hope this helps!

ok