try this snippet to pre-select the current active document folder in an open dialog.
Make sure the active document is saved at least once. Unsaved documents don't have a file path
// preselect folder, set default open folder to active document folder (saved)
var f = app.activeDocument.fullName;
var ff = f.openDlg ("select file from f parent folder", "*.*", false);
//File.openDialog()