if you have vba installed you can try this
(setq D1 "c:\\ dwgname.dwg")
(command "._VBASTMT" (strcat "AcadApplication.Documents.Open \"" D1 "\""))
(setq dwgName "\"C:\\dwgname.dwg\"")
(command "vbastmt" (strcat "AcadApplication.Documents.Open " dwgName))
or you can use Lee Mac's function just place the code in your acaddoc.lsp and use the function call in toolbar button or a pulldown
;; Author: Lee Mac, Copyright © 2011 - www.lee-mac.com ;;
(defun LM:Open ( target / shell result )
(if
(and
(or
(eq 'INT (type target))
(setq target (findfile target))
)
(setq shell (vla-getInterfaceObject (vlax-get-acad-object) "Shell.Application"))
)
(progn
(setq result (vl-catch-all-apply 'vlax-invoke (list shell 'open target)))
(vlax-release-object shell)
(not (vl-catch-all-error-p result))
)
)
)
(vl-load-com)
Example to Open a File Selected by the User
(LM:Open (getfiled "Select File to Open" "" "" 16))
Example to Open a Drawing File at a Specified Location:
(LM:Open "C:\\My Folder\\File.dwg")
Example to Open a Directory:
(LM:Open "C:\\My Folder\\My SubFolder")