What am I missing in my domain mapping setup?

I want to map a TLD (topdomain.com) to a subsite (topdomain.mysite.com) however I am not getting the desired results. Entering topdomain.subsite.com maps to topdomain.com but I get page not found error for topdomain.com
I followed the Usage tab instructions -
Sunrise.php is up to date, and the WP network is on an apache server with a dedicated IP.
I added an A record pointing to this dedicated IP to the DNS records for topdomain.com

Any thoughts what might be causing this?