This should be simple enough.

Code:
Private Sub Command1_Click()
    Dim pos As Integer
    pos = InStrRev(Text1.Text, vbNewLine)
    If pos = 0 Then
        MsgBox "The last line is: " & Text1.Text
    Else
        MsgBox "The last line is: " & Mid(Text1.Text, pos + 2)
    End If
End Sub