The Default Plot to File Path can be set in AutoCAD using the Options Dialog. To do this programmaticaly using ARX, you can use the automation function IAcadPreferencesOutput::put_DefaultPlotToFilePath().
Please find below some code which shows you how to do this programmatically.
#pragma warning( disable : 4278 )
// Makes change to the tlb name based on the AutoCAD version.
// For ex : acax18enu.tlb for AutoCAD 2010/2011 and 2012
// acad19enu.tlb for AutoCAD 2013
#import "acax19ENU.tlb" no_implementation raw_interfaces_only named_guids
#pragma warning( default : 4278 )
#include <acadi.h>
static void AdskMyCommand(void)
{
CWinApp *pApp = acedGetAcadWinApp();
HRESULT hRes;
LPDISPATCH pDisp=NULL;
if(!pApp)
return;
pDisp=pApp->GetIDispatch(TRUE);
if (!pDisp)
return;
CComPtr<AutoCAD::IAcadApplication> pComApp;
hRes=pDisp->QueryInterface(
IID_IAcadApplication,
(void**)&pComApp
);
if (FAILED(hRes))
return;
CComPtr<AutoCAD::IAcadPreferences> pComPref;
hRes=pComApp->get_Preferences(&pComPref);
if(FAILED(hRes))
return;
CComPtr<AutoCAD::IAcadPreferencesOutput> pComOutputPref;
hRes=pComPref->get_Output(&pComOutputPref);
if(FAILED(hRes))
return;
BSTR bstDefaultFilePath;
hRes=pComOutputPref->get_DefaultPlotToFilePath(&bstDefaultFilePath);
if(FAILED(hRes))
return;
CString cstrVal = "C:\\Temp";
BSTR bstNewDefaultFilePath = cstrVal.AllocSysString();
hRes=pComOutputPref->put_DefaultPlotToFilePath(bstNewDefaultFilePath);
if(SUCCEEDED(hRes))
{
acutPrintf
(
ACRX_T("DefaultPlotToFilePath has been changed from %s to %s."),
bstDefaultFilePath,
bstNewDefaultFilePath
);
}
}