I can't connect to Mac with Xamarin Mac Agent from Visual Studio 2015

You have to try delete everything from

%localappdata%\Xamarin\MonoTouch

It helped me, I had the same (or similar) problem today.

I have found the solution about my problem. I have done the following steps:

1. In my Windows: I have removed the content from "%localappdata%\Xamarin\MonoTouch."
2. In Mac terminal: Remove the authorized_keys Mac file ("rm /Users/macmini2/.ssh/authorized_keys" in my case)
3. In Mac terminal: chmod g-w /Users/macmini2/
4. In Mac terminal: chmod 700 /Users/macmini2/.ssh/
5. In Mac terminal: Create a empty file: /Users/macmini2/.ssh/authorized_keys (with vim, for example)
6. In Mac terminal: chmod 600 /Users/macmini2/.ssh/authorized_keys
7. On Visual Studio: Login with Xamarin Mac Agent
8. On Visual Studio: Run the application with [Debug] - [iPhoneSimulator] - [iPhone 5 iOS 8.1]
9. And finally, the simulator runs on Mac computer.

This link helped me.

Xamarin has a Troubleshooting page with large focus on SSH issues:

Connection Troubleshooting

Specifically these two:

Log File Location

Mac – ~/Library/Logs/Xamarin-[MAJOR.MINOR]
Windows – %LOCALAPPDATA%\Xamarin\Logs

The log files can be located by browsing to Help > Xamarin > Zip Logs in Visual Studio.

"Unable to authenticate with SSH keys. Please try to log in with credentials first"

Known cause:

SSH security restriction – This message most often means that one of the files or directories in the fully qualified path of $HOME/.ssh/authorized_keys on the Mac has write permissions enabled for other or group members. Common fix: Run chmod og-w "$HOME" in a Terminal command prompt on the Mac. For details about which particular file or directory is causing the problem, run grep sshd /var/log/system.log > "$HOME/Desktop/sshd.log" in Terminal, and then open the sshd.log file from your Desktop and look for "Authentication refused: bad ownership or modes".

Furthermore there is this section, which does not directly match the error you are seeing but goes into some detail on SSH configuration and diagnosis:

"Couldn't connect to MacBuildHost.local. Please try again."

Reported causes:

Bug – A few users have seen this error message when attempting to log in to the build host using an Active Directory domain user account.

Bug – Some users have seen this error when attempting to connect to the build host by double-clicking the name of the Mac in the connection dialog. Possible workaround: Manually add the Mac using the IP address.

Bug #35971 – Some users have run across this error when using a wireless network connection between the Mac build host and Windows. Possible workaround: Move both computers to a wired network connection.

Bug #36642 – On Xamarin 4.0, this message will appear anytime the $HOME/.bashrc file on the Mac contains an error. (Starting with Xamarin 4.1, errors in the .bashrc file will no longer affect the connection process.) Workaround: Move the .bashrc file to a backup location (or delete it if you know you don't need it).

Limitation – This error can appear if the Mac build host is connected to a router that has no access to the internet (or if the Mac is using a DNS server that times out when asked for the reverse-DNS lookup of the Windows computer). Visual Studio will take roughly 30 seconds to retrieve the SSH fingerprint and eventually fail to connect.

Possible workaround: Add "UseDNS no" to the sshd_config file. Be sure to read about this SSH setting before changing it. See for example http://unix.stackexchange.com/questions/56941/what-is-the-point-of-sshd-usedns-option.

The following steps describe one way to change the setting. You will need to be logged in to an administrator account on the Mac to complete the steps.

    Confirm the location of the sshd_config file by running ls /etc/ssh/sshd_config and ls /etc/sshd_config in a Terminal command prompt. For all of the remaining steps, be sure to use the location that does not return "No such file or directory".

    Run cp /etc/ssh/sshd_config "$HOME/Desktop/" in Terminal to copy the file to your desktop.

    Open the file from your Desktop in a text editor. For example you can run open -a TextEdit "$HOME/Desktop/sshd_config" in Terminal.

    Add the following line at the bottom of the file:

    UseDNS no

    Remove any lines that say UseDNS yes to make sure the new setting takes effect.

    Save the file.

    Run sudo cp "$HOME/Desktop/sshd_config" /etc/ssh/sshd_config in Terminal to copy the edited file back into place. Enter your password if prompted.

    Disable and re-enable Remote Login under System Preferences > Sharing > Remote Login to restart the SSH server.