The command you want to type is:
icomp6 r data1.cab simjp.exe simjp.exe
but that's not the problem - there is another version of i6comp about somewhere - one worked for me while the other didn't. The one I have is a zip file named i6comp02.zip and is about 121Kb in size. Also, if the game is new then i6comp may not be able to edit the cab files cos they've been updated.
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