It's a bit involved... you end up having to repackage the SRPMS in MLNX_OFED_SRC so that you can run mlnx_add_kernel_support.sh. I cannot remember all the steps.
Here is a tarball of the modified MLNX_OFED_LINUX with added support for 2.6.18-371.1.2.el5. Hope this helps.
http://www.uvm.edu/~jtl/MLNX_OFED_LINUX-1.5.3-4.0.42-uvm1-rhel5.10-x86_64.tgz