DNS = Domain Name System. It's a server address that resolves domain names to public IP addresses. This has nothing to do with dyndns, other than how your Elk system will reach dyndns.
I typically use the ones my ISP provides with the assumption that if the DNS is down, the ISP is as well or vice versa. To find out what your current DNS is, open a Command Prompt window (click the Start menu and search for 'cmd'). Once open, enter the command: 'ipconfig /all'. Your DNS server address(es) will be listed.
Having said that, you can also use Google's: 8.8.8.8
If you have Comcast they use: 75.75.75.75 and 75.75.76.76