Hi Steve,
Officially the Multilink Universal FX was introduced at MCU v10.2 version.
So previous version and classic version are not supported by default -> this is including HC12 V5.1.
However ...
Here is patch for Universal FX interface under CW5.1.
Note: The current version of unit_12z.dll is 2.78 (released 5/9/2012)
Regards
Pascal