I still can't find it! Like the man said I don't see a 'more actions' button, either on galaxy or web site...
Go to the game library page via the website, underneath the title of the game there should be a heading that says 'More'. Click that and a sub menu will open and you will find a heading called 'Serial keys'. Click that and they should be there. Hope this helps.
I'm afraid I remain confused. I go to gog.com, and go into my account, and click to see 'My Collection' (I assume that's what you mean by Library?) but 'more' offers me options like set tags, download, but nothing about serial keys. Frustrating.