I went to xd.adobe.com and signed in but I'm not sure what this actually does since I can't see anywhere to view my mockups or any aspect of my account. The only thing I see different about the page when signed in is a "Sign out" button.
Is there a central place where I can see all of my online mockups?
I am not sure I get your question. Did you try downloading Adobe XD from your Creative Cloud desktop application?
See Learn how to download your Adobe Creative Cloud apps for info on downloading Creative Cloud applications.
After you open Adobe XD, use the In App tutorial to get started with using the application. Let me know if you need more assistance, or if I misread your question.
You can go to assets.adobe.com and select "Prototypes" from the left nav bar. You'll get a list of all of the prototypes you've shared.
Hope that helps!