If you are using an MFC Regular DLL, you have to manage module state switching when calling acutPrintf(). You need to do something like this:
void arx_func() {
// Put AFX_MANAGE_STATE in its own scope
{
AFX_MANAGE_STATE(AfxGetAppModuleState());
acutPrintf(L"\nHi!");
} // End of scope
// More of your code
}