In my setup (which works so well - thanks itimpi) the location is:
https://myname.updog.co/_catalog/index.xml
Because, I put the _catalog directory in with the other calibre directories (oops) but it still works.
Your path when adding OPDS will look like:
https://website.updog.com/location-of_catalog/index.xml
So, if your _catalog directory doesn't live in the main website directory (with all the other author directories), you need to change the "location-of_catalog" to its path.
Does that make sense?
Edited to add: itimpi the path to the User Manual is broken on your blog post "Use Updog.co to continue using Dropbox!" (I tried to leave a comment there but...I failed).