The list of Host available for a FileList Variable will not include all the agents that you have in your system.
This will *not* include the RA/SQL Agents (eg: FTP, SQL, etc).
So if your (eg) RA_FTP is installed in (eg) Server_ABC and you want to get the FileList from server_ABC, you won't be able to see RA_FTP from the list of available hosts in a FileList variable.
To use server_ABC in a FileList variable, you'll need to install an OS agent (Windows, Unix). Once you have that setup, then you should be able to select the OS agent installed in Server_ABC in the dropdown list.