Problems when add another shard
First.. You are not adding another shard, just new replica set member. Error means that this (primary) could not connect that "node75" at port 27017 to check that it is live.

Problem is probably that, that this name node75 cannot be resolved. There is no IP address for that name.

Try to add line to /etc/resolv.conf

xxx.xxx.xxx.xxx  node75


where that xxx is IP address of that machine.

Or use IP-address at that rs.add("xxx.xxx.xxx.xxx:27017")

Of course you need to check that firewall (at node75) is open for that port 27017.