You need to download the Toolbox version 1.1.0.0 or later, and add a reference to your project.
Dim s_targetfile As String = My.Computer.FileSystem.SpecialDirectories.Desktop & "\target.pdf"
Dim s_sourcefiles As String() = {My.Computer.FileSystem.SpecialDirectories.Desktop & "\file1.pdf", _
My.Computer.FileSystem.SpecialDirectories.Desktop & "\file2.pdf", _
My.Computer.FileSystem.SpecialDirectories.Desktop & "\file3.pdf"}
Dim mypdftool As New ToolBox.PDFWorker
mypdftool.parm_ApplicationName = "Demo Application"
mypdftool.parm_Author = ""
mypdftool.parm_Keywords = "Demo, PDF Merge"
mypdftool.parm_Subject = "Demo file merged from multiple source files"
Cursor.Current = Cursors.WaitCursor
mypdftool.MergePdfFiles(s_sourcefiles, s_targetfile)
Cursor.Current = Cursors.Default