My first thought is that you should upgrade your System Manager to the latest version (3.1.3). This may simply resolve the issue, and it includes some protocol and security updates that can't hurt. It might even add some functionality, I'm not sure - it's been a LONG time since I used v3.0.
If it doesn't, then I'd suggest making sure that your host can reach the NAS via Port 443 (or 80 if you're still allowing unencrypted connectivity). That's the most likely cause for a "Connection Refused" error. If you cannot reach the NAS via that port, first check to see if the appropriate "httpd" options are enabled on the NAS - see the System Manager "Installation and Setup Guide" and the "Managing Data OnTAP Operating in 7-Mode Using the GUI" guides. If the NAS options are correctly enabled and you still cannot reach it via port 443 or 80, then it smells like a firewall issue.
Good luck, I hope you get SM connected and working soon!