save this in your digitalcoin.conf file placed in your application data directory. I am not proficient with knowledge when it comes to Mac, but place it where ever your blockchain files are stored.
EDIT: Make sure your firewall isn't blocking the digitalcoin-qt software too. Unblock its ports.
On Mac, it's ~/Library/Application Support/digitalcoin/
Here's what I have for nodes. Like jaywaka2713 said, make sure port 7999 is open/forwarded on your firewall.
addnode=24.138.46.123
addnode=46.163.105.201
addnode=50.116.22.43
addnode=175.156.214.215
addnode=174.50.248.26
addnode=175.156.211.216
addnode=71.187.252.146
addnode=71.77.233.192
addnode=78.21.229.230
addnode=124.169.205.168
addnode=50.137.233.14
addnode=124.184.252.156
addnode=101.98.152.38
addnode=37.59.68.144
addnode=69.156.168.41
addnode=115.132.91.41
That is an impressive node list! Going to have to expand mine lol.