There is an HDLC_test.c sample that comes with the Dynamic C 9, you can use this sample as a reference to implement
HDLC application for Rabbit 6000. In Dynamic C 10, we have provided the library file HDLC_PACKET.LIB related to HDLC, you can find at the below directory
C:\DCRABBIT_10.72\Lib\Rabbit4000\Peripherals
Please go through section 20.3.2 HDLC Mode (Page number: 205) from the Rabbit 5000 processor reference manual at the below link.
Here mentioned that what are the steps you need to do in the HDLC mode.
http://ftp1.digi.com/support/documentation/90001108_H.pdf
Also please go through the following thread at the Digi forum, .
http://www.digi.com/support/forum/35992/how-do-go-about-configuring-sdlc-on-serial-port-of-rabbit-6000