How do I give accessibility permission to my AppleScript and System Events in Mavericks?
Your problem is independent of the accessibility permission. 

Replace your click line with something like click window 1 and it will ask you to allow or deny access to the accessibility features. 

There is a problem with click at and I don't know what it is.  

Edit : My conclusion after some tests : click at is broken. 
The release note of AppleScript updated with the release of Mavericks does not state some kind of removal of this feature. Some that might be a bug. 

I submitted a bug report to Apple. 

Edit : The issue has been fixed on Yosemite, click at works well again !