public void AddButton(
    string name,
    RectangleF rect,
    string toolTip = null
)

Public Sub AddButton(
    name As String,
    rect As RectangleF,
    toolTip As String = Nothing
)

using DevExpress.Pdf;
using System.Drawing;
//...

static void Main(string[] args)
{
    using (PdfDocumentProcessor processor = new PdfDocumentProcessor())
    {
        // Create an empty document.
        processor.CreateEmptyDocument("..\\..\\Result.pdf");

        // Create graphics and draw a radio button field.
        using (PdfGraphics graphics = processor.CreateGraphicsPageSystem())
        {
            DrawRadioButtonGroupField(graphics);

            // Render a page with graphics.
            processor.RenderNewPage(PdfPaperSize.Letter, graphics);
        }
    }
}

static void DrawRadioButtonGroupField(PdfGraphics graphics)
{
    // Create a radio group field.
    PdfGraphicsAcroFormRadioGroupField radioGroup =
         new PdfGraphicsAcroFormRadioGroupField("First Group");

    // Add the first radio button and specify its location.
    radioGroup.AddButton("button1", new RectangleF(0, 0, 20, 20));

    // Add the second radio button.
    radioGroup.AddButton("button2", new RectangleF(0, 20, 20, 20));

    // Specify selected index, style and appearance parameters.
    radioGroup.SelectedIndex = 1;
    radioGroup.ButtonStyle = PdfAcroFormButtonStyle.Circle;
    radioGroup.Appearance.BackgroundColor = Color.Aqua;
    radioGroup.Appearance.BorderAppearance =
         new PdfGraphicsAcroFormBorderAppearance()
    {
        Color = Color.Red,
        Width = 3
    };

    // Add the field to graphics.
    graphics.AddFormField(radioGroup);
}

Imports DevExpress.Pdf
Imports System.Drawing
'...
Shared Sub Main(ByVal args() As String)
    Using processor As New PdfDocumentProcessor()

        ' Create an empty document.
        processor.CreateEmptyDocument("..\..\Result.pdf")

        ' Create graphics and draw a radio button field.
        Using graphics As PdfGraphics = processor.CreateGraphicsPageSystem()
            DrawRadioButtonGroupField(graphics)

            ' Render a page with graphics.
            processor.RenderNewPage(PdfPaperSize.Letter, graphics)
        End Using
    End Using
End Sub

Private Shared Sub DrawRadioButtonGroupField(ByVal graphics As PdfGraphics)

    ' Create a radio group field.
    Dim radioGroup As New PdfGraphicsAcroFormRadioGroupField("First Group")

    ' Add the first radio button and specify its location.
    radioGroup.AddButton("button1", New RectangleF(0, 0, 20, 20))

    ' Add the second radio button.
    radioGroup.AddButton("button2", New RectangleF(0, 20, 20, 20))

    ' Specify radio group selected index, style and appearance parameters.
    radioGroup.SelectedIndex = 1
    radioGroup.ButtonStyle = PdfAcroFormButtonStyle.Circle
    radioGroup.Appearance.BackgroundColor = Color.Aqua
    radioGroup.Appearance.BorderAppearance = New PdfGraphicsAcroFormBorderAppearance() With
    {
        .Color = Color.Red,
        .Width = 3
    }

    ' Add the field to graphics.
    graphics.AddFormField(radioGroup)
End Sub