To follow-up on Tim's advice, most likely the issues stems from this line in your Makefile:
This will, in fact, link the 32-bit version of the Intel MPI Library (or -lmpi). To instead use the 64-bit versions, you need to do:
In general, instead of manually defining your lib path, we recommend you source the appropriate
mpivars scripts from the bin/ directory (for 32-bit executables) and bin64/ directory (for 64-bit executables) if you're running on an Intel 64 machine.
Hope this helps. Let us know if you have other questions.