There is a link on the left hand side of the ZipTie Client that says "Configure Credentials". This is where you tell ZipTie which usernames, passwords, etc, that your devices use.

See this doc link for more info.
http://docs.ziptie.org/doku.php?id=ugdoc:users_guide#credential_and_prot...

I have created a routers group, and added a router into the Addresses section. I was referring moreover to the Credentials section where you specify the actual usernames and passwords/enable username and enable password.

With the Cisco routers if you setup your vty interface as follows:

line vty 0 15
password myPassword
login

You are prompted for what is known as a telnet password, then if you have your enable or enable secret set you also have that password for enable mode. There is no option in ziptie for the telnet password.

If you created a new address group, then you also need to add a new credential group at the bottom of that editor. In there you'll find a place to fill out a "Username", "Password" and an "Enable Password (secret)". You're not using a local account so just fill in the "Password" and the IOS adapter will know what to do with it.