5
The URL to the front page of your XOOPS site should work, provided that the block is visible on the front page, the block is visible to anonymous users, and you placed the correct code in the block.
You should be able to check the above by browsing to your site's URL while not logged in, and viewing the HTML source for the page.
If all of the above is correct, and the web ring doesn't recognize the code, you should contact the ring manager and have him look at the page.
I used to manage a web ring, but haven't kept up with that.