I'm a relatively new user myself, but because I figured out this lately myself, I'll chance replying here.
You do have to buy Diarium from the Microsoft Store, and then set up a cloud sync through an account of yours -- the options are:
OneDrive / google drive / Dropbox / WebDav / iCloud
I set it up recently via dropbox -- to be honest it was so easy I forget the exact steps, so I think it shouldnt be a problem no matter which option you choose.
I'm very happy with it, and feel it's worth the double-purchase. The logic for the double purchase was explained somewhere in the diarium website, just look for the FAQ's -- there's more info there too re syncing (not sure if I can post a link as a new forum member)