go to tools preferences, advanced, isoviewfile formats and make sure that cgm is in the list. May need to reinstall isoview from the Arbortext editor install folder.
Did you recently upgrade versions may need to rerigister your isoview.
Start > Run > cmd
In the DOS window, navigate to the install path for ArbortextIsoView 7.3, then enter
regsvr32 IsoViewX73.ocx
or
regsvr32 IsoViewX73_x64.ocx
Bryon