Okay! Disregard the links at the bottom of this post as @HappyMapper just made the above post before I could hit "Post." (But that was how I believe I got where I was going. Either that or I clicked on the link to GlobalXplorer somewhere in a forum post. Now I'm starting to confuse myself.
Actually, I accidentally clicked on an old link in my multi-clipboard that was for the Population Mapping in the Congo. A window popped up to inform me that this campaign was over. It had a button to click on to "Find a New Campaign" which I clicked. I was taken to Tomnod's homepage and there it was sitting right at the top - Worldwide: Urban Mapping. I also tried simply going to the GlobalXplorer homepage and got the same campaign, but was asked to login. (That's I asked @HappyMapper about setting up a new account or using my Tomnod credentials.)
This may have been the case before the new Tomnod campaign (Peru) was put up however. Now when I go to Tomnod's homepage, it shows the "Peru: Our Shared Human History." I guess you'll have to go right to the GlobalXplorer homepage to login their for their campaign "Worldwide: Urban Mapping." OR... Just go to
https://www.tomnod.com/campaign/campaign_1683/question/10000 and that should get you started - unless the problem mentioned earlier is still being addressed.