Digidesign has changed the method of updating the firmware on the SyncIO with v7.3 or later.

To update your SYNC I/O firmware:
1) Connect the SYNC I/O to the Digiserial port of the core card in your Pro Tools system.
2) Pro Tools software must NOT be running.
3) Launch the Digitest application.

Mac HD > Applications > Pro Tools > Pro Tools Utilities > Digitest
-or-
Local Disk C: (system drive) > Program Files > Digidesign > Pro Tools > Pro Tools Utilities > Digitest

4) In the Digitest application, click on the Synchronizer Firmware Tab, and choose the slot containing the HD Core Card in the pop-up menu.
5) Select DigiSerial Port and click Begin Update.
6) Navigate to the Sync I/O Firmware file

Mac HD > Applications > Pro Tools > Pro Tools Utilities > SYNCfirmware111.SYC
-or-
Local Disk C: (system drive) > Program Files > Digidesign > Pro Tools > Pro Tools Utilities > SYNCfirmware111.SYC

7) Follow the onscreen instructions to complete the update.
Note: If you receive a message that the SYNC I/O firmware update aborted, do the following:
1) Make sure the SYNC I/O is properly connected to the DigiSerial Port on the HD Core card and that the slot containing the HD Core card is selected in DigiTest.
2) Power cycle the SYNC I/O and repeat the update steps.
(Taken from the "Read Me" which documents compatibility details and known issues for using SYNC I/O with Pro Tools 7.3.0)
www.digidesign.com