openmp on visual studio express 2012

I've activated openmp in the c++ project (Properties->C/C++->Language->Open MP Support).

I'm doing a porting of an existing project with parallelism code like the following:

                #pragma omp parallel for schedule( dynamic, 1 )
                for( signed i = 0; i < nsize; ++ i ){

I can be wrong, but from the taskmgr it seems that only one core is getting to 100%: any suggestion ?

Thanks