Refer to the C5000 Teaching ROM chapter 6, chapter 20 and this wiki page:
http://processors.wiki.ti.com/index.php/Porting_C5000_Teaching_ROM_to_C5535_eZdsp
It is for the C5535 eZDSP but C5515 eZDSP and C5535 eZDSP are very similiar. Chapter 20 teaches upgrade to C5515.
Also refer to the C5535 TRM's FFT chapter. There is a link to download the FFT example code.
Project collateral and source code discussed in this application report can be downloaded from the
following URL: http://www-s.ti.com/sc/techlit/sprabb6.zip.
Regards.