I deleted WordPress and started afresh and even created another new app on Facebook but still the issue remains.
I have set up a Facebook app which points to my WordPress site.
All parts of the site work, except I cannot click the register or login buttons.
If I log in from outside Facebook and refresh the Facebook app/page then I appear logged in and can do what I want, including log out.
Any ideas please? Unfortunately, most Google searches think I am talking about third party Facebook connect plugins, which I’m not.
The page in question is here, which is just looking at http://www.costablancaclassifieds.com.
From what I can tell Brad, this seems to be less of a FB problem and more of a WP cookie issue. I tried framing your site in with an iframe and it reacted similarly to the problem you’re experiencing. Looked around a bit and found others with similar problems; issue being that when you’re serving a WP site to another domain via an iframe the cookies aren’t able to be set properly.
Here’s one forum topic I found: http://wordpress.org/support/topic/running-wordpress-in-an-iframe-cross-domains
It’s not good news, but hopefully it helps you find a solution or at least get some level of closure. I’ll try to think of a creative solution for your situation.