Problems with Helix MP3 Decoder

Hi everybody, i'm student on the UPV of Valencia and i'm trying to port 
the Andreas Schwarz MP3 Player to a ST M3 Cortex. I have a problem when 
i compile the code with keil:

Error[Li005]: no definition for "xmp3_UnpackFrameHeader" [referenced 
from C:\IAR Systems\Embedded Workbench 5.4\arm\examples\ST\STM32F10x\
IAR-STM32F103ZE-SK\ARMP3\Flash Debug\Obj\mp3dec.o]

Can someone give me help and tell me where can be the problem?
Thank's everybody.
Excuse me for my english, it's very bad.