private void RadWizard1_NextButtonClick(object sender, WizardEventArgs e)
{
    if (Page.IsValid)
    {
        int activeStepIndex = RadWizard1.ActiveStep.Index;
        var wizstep = RadWizard1.WizardSteps[activeStepIndex];
        wizstep.Enabled = true;

        for (var index = 0; index < wizstep.Controls.Count; index++)
        {
            var ctrl = wizstep.Controls.Item[index] as WebControl;
            if (ctrl != null)
                ctrl.Enabled = true;
        }
    }
}

Private Sub RadWizard1_NextButtonClick(sender As Object, e As WizardEventArgs) Handles RadWizard1.NextButtonClick
    If Page.IsValid Then
        Dim activeStepIndex As Integer = TryCast(sender, RadWizard).ActiveStep.Index
        Dim wizstep = RadWizard1.WizardSteps.Item(activeStepIndex)
        wizstep.Enabled = True

        For index = 0 To wizstep.Controls.Count - 1
            Dim ctrl = TryCast(wizstep.Controls.Item(index), WebControl)
            If ctrl IsNot Nothing Then
                ctrl.Enabled = True
            End If
        Next
    End If
End Sub