The same thing happened with me last night. I had problems with both GK1 and GK2 appearing on the Downloader queue last week, but they eventually appeared after about a minute. However, I could not get it to download GK3. Clicked endlessly on "Add to Downloader" on "My Account", refreshed the queue several times, restarted GOG Downloader, clicked on "Add to Downloader" from other browsers other than IE, tried all that for about a couple of hours and nothing. Ended up downloading it through IE.
Namur: Yeah, something's up with that file. I can't add it to the queue either, the 'refresh queue' spininng icon doesn't stop. I can add other games to the queue though, so i think the problem might be specific to that one file.
I've downloaded GK3 last night with my browser (IE8) and it downloaded fine, so i guess you can use your browser.