DwgThumbnail
It is almost certain that the DwgThumbnail control you used is a 32-bit component, which you cannot use it in 64-bit AutoCAD (I assume you use 64-bit AutoCAD 2015, but if you still use 32-bit AutoCAD, then that control should have worked for you).
Whether you use 32 or 64 bit AutoCAD, though, you can forget the old DwgThumbnail control. There is easy way to access the thumbnail as long as your AutoCAD version is not too old (acad2010 or newer).
If your are doing AutoCAD add-in with .NET API, you can access drawing thumbnail by Database.ThumbnailBitMap property.
If you are instested to get drawing thumbnail without opening AutoCAD, then you can look at code sample here:
On the Win Form, you'd have a Picture Control, right? Then you retrieve the Thumbnail as System.Drawing.Image from an opened drawing (or, you can read a drawing as side Database, so that you can get the Thumbnail).
Ideally, you'd have a helper class to get the image, something like:
public class CadHelper
{
public static System.Drawing.Image GetThumbnail(Document dwg)
{
//Retrieve thumbnail froom already opened drawing document
}
public static System.Drawing.Image GetThumbnail(string dwgFileName)
{
//oepn a drawing file as side database and retrieve the Thumbnail
}
}
Then you can do this in the form's code behind:
MyPictureControl.Image=CadHelper.getThumbnail(...);
You DO NOT use DwgThumbnail control, because it is 32 bit and your AutoCAD is 64-bit.
If you read my previous reply carefully, you'd have seen that I said to use Win Form's PICTURE control.
Well,
public static System.Drawing.Image.GetThumbnail(...)
{
//the code could be as simple as this:
Database db=ApplicationDocumentManager.MdiDocument;
return db.ThumbnailBitmap;
}
is a method YOU are going to write to retrieve Thumbnail from a drawing database. it should return an System.Drawing.Image object that you can assigned to a picture control (OK, it is PictureBox control, to be technically correct).
Here is the code I've been using.. I would LOVE to give props to the source, but I don't remember what it is 
You feed it the DWG path and it returns a Bitmap image (if found) BMP for 2012 and earier, PNG for 2013 and later
Imports System.IO
Function GetBitmap(fileName As String) As Bitmap
Using fs As New FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
Using br As New BinaryReader(fs)
fs.Seek(&HD, SeekOrigin.Begin)
fs.Seek(&H14 + br.ReadInt32(), SeekOrigin.Begin)
Dim bytCnt As Byte = br.ReadByte()
If bytCnt <= 1 Then
Return Nothing
End If
Dim imageHeaderStart As Integer
Dim imageHeaderSize As Integer
Dim imageCode As Byte
For i As Short = 1 To bytCnt
imageCode = br.ReadByte()
imageHeaderStart = br.ReadInt32()
imageHeaderSize = br.ReadInt32()
If imageCode = 2 Then ' BMP Preview (2012 file format)
' BITMAPINFOHEADER (40 bytes)
br.ReadBytes(&HE)
'biSize, biWidth, biHeight, biPlanes
Dim biBitCount As UShort = br.ReadUInt16()
br.ReadBytes(4)
'biCompression
Dim biSizeImage As UInteger = br.ReadUInt32()
'br.ReadBytes(0x10); //biXPelsPerMeter, biYPelsPerMeter, biClrUsed, biClrImportant
'-----------------------------------------------------
fs.Seek(imageHeaderStart, SeekOrigin.Begin)
Dim bitmapBuffer As Byte() = br.ReadBytes(imageHeaderSize)
Dim colorTableSize As UInteger = CUInt(Math.Truncate(If((biBitCount < 9), 4 * Math.Pow(2, biBitCount), 0)))
Using ms As New MemoryStream()
Using bw As New BinaryWriter(ms)
bw.Write(CUShort(&H4D42))
bw.Write(54UI + colorTableSize + biSizeImage)
bw.Write(New UShort())
bw.Write(New UShort())
bw.Write(54UI + colorTableSize)
bw.Write(bitmapBuffer)
Return New Bitmap(ms)
End Using
End Using
ElseIf imageCode = 6 Then ' PNG Preview (2013 file format)
fs.Seek(imageHeaderStart, SeekOrigin.Begin)
Using ms As New MemoryStream
fs.CopyTo(ms, imageHeaderStart)
Dim img = Image.FromStream(ms)
Return img
End Using
ElseIf imageCode = 3 Then
Return Nothing
End If
Next
End Using
End Using
Return Nothing
End Function