The slave machine must be accessible from the Internet. If it is directly connected to the Internet it should have a DNS entry, which would either be something you had to configure yourself; something your network administrator configured; or maybe something your ISP provides for you. If you don't have a DNS entry for the machine, you can use the machine's IP address instead.
If you are behind a NAT router of some sort, you're going to need to forward the SSH port (22) to the internal machine. You'd then want to use your NAT router's IP for the slave machine entry.
If it prompted you for the password then you are connecting properly to the remote machine and it is responding. If it is then just sitting there it could be an issue with the SSH daemon on the remote machine authenticating your connection. Unfortunately troubleshooting SSH is really beyond the scope of this article.
In my personal experience, sshd will often hang if the remote machine is not able to do a reverse lookup on the IP address that is connecting. If you are connecting from a machine with an RFC1918 IP (i.e. 192.168.x.x/24, 10.x.x.x/8, 172.16.x.x/16) and do not have reverse DNS entries for those IPs, that might be at fault. In that case however, it will hang for maybe 30 seconds (until the lookup times out) and then it should let you in. If it's still not letting you in, I'm honestly not sure what would be causing the problem. You might have some luck searching Google for "BSD sshd refuses connection" or "BSD ssh connection times out". Good luck!