Hi Sebastian,
You don't need an external SSH client in order for the node to work. KNIME/Eclipse comes with its own ssh client implementation.
You do need to make sure the configuration is correct though: In the preferences (File->Preferences, Filter text: "SSH2"), you need to point it to the appropriate "SSH2 home" directory that must contain a "known_hosts" file. (Unfortunately it defaults to $user/ssh - usually it is $user/.ssh). It will only connect to hosts contained in the known_hosts file.
The best way to add a host to this file used to be to connect with putty to the host (for Windows users). Putty asks you if you want to store the key of the host - and if you agree, it stores it in the known_hosts file. It used to. Apparently the latest version of putty stores it now in the Windows registry... so that is out the window.
On Linux a successfull ssh connection still stores the host key in the $home/.ssh/known_hosts and you can use this file also under Windows and point KNIME to it.
If you change the SSH2 home in the preference page, close and re-open the preference page and check the "SSH2/Known Hosts" tab. The host you want to connect to should appear there.
And, for your last question: you specify the user that should connect to the remote machine in the dialog of the ExtSSHTool node. Username and Password are the fields that should be filled with the credentials, the passphrase for the private key file can be left blank then.
Hope that helps, best regards,
- Peter.