I believe this is what you are looking for:

private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == (char)Keys.Enter)
    {
        treeView1.Nodes.Add(textBox1.Text);
    }
}

(P.S. how come my code doesn't appear nice and color-coded? Is it an IE-only thing?)