I've found a fix to the problem where the R18 Help Docs don't launch from within CATIA, giving the following error
"No document is associated to the current command."
The fix was to add a line to the Environment File. There are some commented out lines at the start of the file. Within this area make sure you have the ARGS line as shown below
!----------------------------------------------------------
! DASSAULT SYSTEMES - V5 ENVIRONMENT FILE
!----------------------------------------------------------
! MODE : Global
! TYPE : CATIA
! TMSTMP : 1234567890
! ARGS : -cs CATIA -a global -icon no -menu yes -tools
!----------------------------------------------------------
It's actually the "-cs CATIA" bit that does the trick.
Now the help docs open exactly as expected within CATIA.
Hope this is useful.