I'm not familiar with the opencores Mac, but are you getting this error message from the software? In that case your problem doesn't come from the .sdc file.
The timing constraints that you set in the .sdc files set the timing requirements on the hardware level. Timing errors on that level are detected during compilation of the Quartus project, in the TimeQuest tool. If you are getting this error message from the embedded software, it probably means that you have a runtime timing problem, i.e. that your software is too slow to cope with the incoming traffic, and the driver drops some frames. In that case you should rather have a look on how you can optimize your design, either on the hardware level (increasing the system frequency, using a memory cache, or on-chip memory buffers or tightly coupled RAMs) or on the software level (compiler optimizations, code optimization).
