You are adding the radio button dynamically during the execution of your program so it only exists while the program is running. Because of that, it is unknown to the Windows Form Designer. Rather than dynamically adding controls while the program is executing, use the Windows Form Designer in Visual Studio to design the form. The Windows Form Designer will generate all of the code to properly instantiate the radio button.
This is an example of what Windows Form Designer would generate for a radio button control:
Friend WithEvents rbFuture As System.Windows.Forms.RadioButton
Me.rbFuture = New System.Windows.Forms.RadioButton()
Me.rbFuture.Location = New System.Drawing.Point(8, 24)
Me.rbFuture.Name = "rbFuture"
Me.rbFuture.Size = New System.Drawing.Size(112, 16)
Me.rbFuture.TabIndex = 2
Me.rbFuture.Text = "Days in the future"