You need to set the child Form's MdiParent Property to your MdiContainer. This example assumes two Forms one named Form1 and the other named Form2. All properties are being set programmatically.
Public Class Form1
Public Sub New()
InitializeComponent()
Me.IsMdiContainer = True
Dim frm2 As Form2 = New Form2
frm2.Owner = Me
frm2.MdiParent = Me
frm2.Show()
End Sub