My understanding is that there isn’t a single Inform 6 IDE, the way there is for I7 (and no, the I7 IDE won’t know what to make of I6 source; I suppose you could just copy and paste it into the IDE and mark it all as an I6 inclusion, but that sounds really messy and unhelpful).
There was this semi-recent thread where folks shared their tools for working with Inform 6 code that might be helpful for identifying some possible options – maybe one of them would be a good fit?