Larger nations have to pay more for infrastructure and technology then what smaller nations due. Donations give them gains in these areas that would cost them many millions. For a small nation the return of a donation is not that significant, so the larger nation pays you in CN money to make a RL $Cash donation to the makers of cybernation in the name of the larger nation. You do this by viewing the larger nations page and clicking on make donation in the upper right field.
You could make a donation towards your own nation however as a smaller nation it is much more valuable to sell it for 18 million CN money.