Paypal doesn't put very much in the way of showing that there is a place where you can leave a note for who a donation is for. When you are checking out, there are roughly 3 steps that you have to do. Right before you check out, after you entered your donation amount, and verified all your information, there is the screen that says Donate xxxx amount now or something like that.

On that page there is a + button where it will create a text box where you can leave a note for us. Included is a picture of what the screen looks like and where the button is located.