I'm using the theme 'Colors' and I would like to add a box at check out where people can leave a message (when it's a present), but can't seem to find it!
I do see it when i just click 'card' (and it pops up at the left)
I can't see it when i just click 'buy' at the product page.
How can i add this?
You can not add such box on shopify checkout because you can not edit it. You can add note box on cart page or popup.
If you want to add note box on cart page or popup then first check in your theme editor
1. Open theme editor->theme setting->find cart option and check if you have option for enable disable for note or open cart page from top drop down and do same.
If you have not option then you have to add textarea in cart popup file with name="note" field.