Just a quick FYI. My partner enabled the plugin and got a session_start error, trying to open a tmp file, permission denied.
File is constant-contact-api/functions.php and the error occurred on line 5.
This seemed like a WP 3.5 issue I'd heard of. They changed the handling of temp files, striving to use the system default. I used their standard "roll-back" fix, see the 3.5 Master List for details. This fixed it right up.
Hope this helps someone.