I found a good answer and there was a pop-up message "do not forget to vote...". So yes, I did not forget, but could not find a way to vote on the answer? Related question: If you bother to put up a reminder message why not put the Vote button there or put it on the web page so that it is not buried there and would be difficult to find. Make it intuitive, please.
4Questions about the site should be posted in here, not the main site. That's just about Ubuntu.– Oli ModDec 16, 2015 at 15:59
4askubuntu.com/tour and askubuntu.com/help/privileges/vote-up– kosDec 16, 2015 at 16:02
On the left hand side of any answer, there's a positive or negative number which indicates the total of how people have voted. There is an arrow button above and below this number which you click to indicate an upvote or downvote. You can use these arrow button once you've gained enough reputation to cast votes. This is all explained clearly on the Tour Page.