MPI (Message Passing Interface) is the de facto standard in High Performance Computing. By using some MPI-2 new features, such as the dynamic creation of processes, it is possible to implement highly efficient parallel programs that can run on dynamic and/or heterogeneous resources, provided a good schedule of the processes can be computed at run-time. A classical solution to schedule parallel programs on-line is Work Stealing. However, its use with MPI-2 is complicated by a restricted communication scheme between the processes: namely, spawned processes in MPI-2 can only communicate with their direct parents. This work presents an on-line scheduling algorithm, called Hierarchical Work Stealing, to obtain good load-balancing of MPI-2 programs that follow a Divide & Conquer strategy. Experimental results are provided, based on a synthetic application, the N-Queens computation. The results show that the Hierarchical Work Stealing algorithm enables the use of MPI with high efficiency, even in parallel dynamic HPC platforms that are not as homogeneous as clusters.