Locate the ENCOM.PRJ projection text file in the following location:
C:\Program Files\Encom/Projections
and open it in a text editing program (eg. Notepad). Add the projection string, such as:
"--- Australian Map Grid (AGD 66) ---"
"AMG Zone 54 (AGD 66)\p20254", 8, 12, 7, 141, 0, 0.9996, 500000, 10000000
and resave the ENCOM.PRJ file in the same location.