See the “+” on the top left of the white (main) window? That’s how you create new notes. Once you’ve entered your account credentials (which is required for you to be able to sync notes across multiple computers), you’ll be able to create notes as desired: If you’re on a more pristine PC computer then you’ll have to enter both your email address and password. Since my computer is already logged in to my account, it shows that same account here. The first time you run it, you’ll be prompted to log in to your account as shown: It’s already installed on your computer so you can simply choose “Open” to launch the program. Make sure that you choose the one listed as “Trusted Microsoft Store app”. To start, make sure you’re running the correct program by doing a Win10 Cortana search for “sticky” (or, in my case, “stickie” which worked just as well). This is actually one place where Microsoft Windows wins out versus the Mac system because in Win10, Sticky Notes does indeed sync through OneDrive, and it’s easy and seamless! The Mac “Notes” program does, through iCloud, and there are also third party apps like Evernote that offer multi-platform synchronized notes, but Stickies? That can’t be what he’s using if it is working across computers.

I have to admit I’m a bit confused about your question because while MacOS X “Stickies” offers a simple and darn useful app, it doesn’t actually sync across computers.

Simple sticky notes synchronize