using System.Drawing.Design;
using DevExpress.XtraReports.UI;
using DevExpress.XtraReports.UserDesigner;
// ...

private void button1_Click(object sender, EventArgs e) {
    // Create a new End-User Designer.
    XRDesignForm designForm = new XRDesignForm();

    // Create a report instance.
    XtraReport1 report = new XtraReport1();

    // Handle the event when the report is loaded into the End-User Designer.
    report.DesignerLoaded += new DesignerLoadedEventHandler(report_DesignerLoaded);

    // Load the report into the designer. 
    designForm.OpenReport(report);

    // Show the End-User Designer window.
    designForm.Show();
}

private void report_DesignerLoaded(object sender, DesignerLoadedEventArgs e) {
    IToolboxService ts = (IToolboxService)e.DesignerHost.GetService(typeof(IToolboxService));

    // Add a custom control.
    ts.AddToolboxItem(new ToolboxItem(typeof(MyControl)));
}

// A custom control.
public class MyControl : XRControl {
    // ...
}

Imports System.Drawing.Design
Imports DevExpress.XtraReports.UI
Imports DevExpress.XtraReports.UserDesigner
' ...

Private Sub button1_Click(ByVal sender As Object, _ 
ByVal e As EventArgs) Handles button1.Click
    ' Create a new End-User Designer.
    Dim designForm As New XRDesignForm()

    ' Create a report instance.
    Dim report As New XtraReport1()

    ' Handle the event when the report is loaded into the End-User Designer.
    AddHandler report.DesignerLoaded, AddressOf report_DesignerLoaded

    ' Load the report into the designer. 
    designForm.OpenReport(report)

    ' Show the End-User Designer window.
    designForm.Show()
End Sub

Private Sub report_DesignerLoaded(ByVal sender As Object, _ 
ByVal e As DesignerLoadedEventArgs)
    Dim ts As IToolboxService = _ 
    CType(e.DesignerHost.GetService(GetType(IToolboxService)), IToolboxService)

    ' Add a custom control.
    ts.AddToolboxItem(New ToolboxItem(GetType(MyControl)))
End Sub

' A custom control.
Public Class MyControl
    Inherits XRControl
    ' ...
End Class