MS Access Question: how to launch file explorer

Depends on what you want to do.  If you want to select a file to be used by your application, or select a folder to be used to store a file you are exporting from your applicaiton, you will need to use the FileDialog object.  There are numerous examples here in EE about how to use the FileDialog feature.  I suggest you search on that term.

Very valid point Dale!  

If this is to allow the user to select a file and return the filename to the application, then you should be looking at using the FileDialog, see: https://www.devhut.net/2016/10/04/late-binding-the-filedialog/

By using the Shell method:

Shell "explorer.exe ""C:\Temp\"""

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E.g. as method do handle the path escaping:

Option Compare Database
Option Explicit

Public Sub ExplorerOpenDirectory(ByVal CPath As String)

  Const DELIMITER As String = """"
  Const EXPLORER As String = "explorer.exe ""?"""

  Shell Replace(EXPLORER, "?", Replace(CPath, DELIMITER, DELIMITER & DELIMITER)), vbNormalFocus

End Sub

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And here a Windows API variation : http://access.mvps.org/access/api/api0001.htm