Aditya,
In no loopback mode, first check if the switch of 6678 EVM can receive packets, you can look at the memory 0x2090b00 and 0x2090c00 range, those registers are explained in table 3-2 of http://www.ti.com/lit/ug/sprugv9c/sprugv9c.pdf.
Then, what is the destination MAC address your external device sent to DSP, you can find it via Wireshark. In the pa_mgmt.c, there is a function called Add_MACAddress(), the line:
{ 0x10, 0x11, 0x12, 0x13, 0x14, 0x15 },
/* Destmac */
should match the dst MAC sent from your external device. Otherwise, PA will discard it.
Regards, Eric