You can easily construct a tree made of TreeNode objects yourself by calling Ftp object's GetList method repeatedly. Just use a method like this one:

    public static TreeNode GetListTree(Ftp ftp, string folder)
    {
        // create a tree node
        TreeNode node = new TreeNode(folder);

        // save current folder
        string originalFolder = ftp.GetCurrentDirectory();

        // change current directory to the next folder
        ftp.ChangeDirectory(folder);

        // get list of files in the current folder
        FtpList list = ftp.GetList();

        // iterate through the list of files
        foreach (FtpItem item in list)
        {
            TreeNode subNode;
            if (item.IsDirectory)
            {
                // if an item is a directory, construct
                // a tree for it recursively
                subNode = GetListTree(ftp, item.Name);
            }
            else
            {
                // if an item is not a directory,
                // just add it to the tree
                subNode = new TreeNode(item.Name);
            }

            // add the subnode to current node's nodes
            node.Nodes.Add(subNode);
        }

        // go back to the original folder
        ftp.ChangeDirectory(originalFolder);

        return node;
    }

Call this on the folder you wish to appear in the TreeView and add the result to TreeView's Nodes collection.