Take a look at the NSOpenPanel class, I think that is a good starting point:
NSOpenPanel* openDlg = [NSOpenPanel openPanel];
[openDlg setCanChooseFiles:YES];
[openDlg setAllowsMultipleSelection:NO];
[openDlg setCanChooseDirectories:NO];
if ( [openDlg runModalForDirectory:nil file:nil] == NSOKButton )
{
....