i understand your problem....
you have to use double Data Type on the place of Integer
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
ListBox1.Items.Add(TextBox1.Text)
Dim total As Double
For Each Str As String In ListBox1.Items
total = total + CDbl(Str)
Next
TextBox2.Text = total
End Sub
End Class
Now you can check your solution