Yep...I know the answer to this b/c I did the same thing yesterday. You need to download a radio and install it (you can do this after you flash to Synergy). To execute:
* download the radio of choice, rename to PG05IMG.zip and place on the root of the SD card (not in a folder)
* power the phone off
* hold power and volume down
* it will boot into HBOOT
* it will check for the file.
* it will ask you to hit up volume to apply or down to cancel.
* hit volume up.
* file will flash
* tap power to boot the phone when it tells you to
* phone will boot up and you're good to go!
* Be sure to remove the file from your SD card after the fact to avoid screwing yourself later.
I should also say there are probably other ways to do this, but this is what I discovered in searching forums for a good period of time yesterday. Being precise in google searches is key.