Matt,
Welcome to the TI E2E forum. I hope you will find many good answers here and in the TI.com documents and in the TI Wiki Pages. Be sure to search those for helpful information and to browse for the questions others may have asked on similar topics.
This is really a question for the compiler team to confirm instead of this C67x Single Core DSP Forum. This thread will be moved there this time for your convenience.
From an old readme.txt that I found for one of the Compiler v6.0 releases, it looks like _mpyidll was added for the C Compiler v6.0.1 and newer.
Which version of the compiler are you using? I assume it is older than that, but I am not sure whether we made that update before moving to CCSv4 or not.
Please consider upgrading to CCSv5.4, if possible.
Regards,
RandyP