The first Hidden Object Puzzle by the fence (with the mailbox and some clothing ) should be easy enough to get this unlocked. The line that is bright blue means that you have to interact with an item (in this case you need a stamp) so its naturally to click on the mailbox. You will need to center the cursor over the items a little when clicking the items though, but with the 4 error margin you should be safe. If you missed on the first puzzle you can try the next one or just start a new game since its just 2 screens away