Since Hi8 is an analog format, you have a hardware problem, not a software problem. Perhaps some later-model Hi-8 camcorders had a Firewire (I think Sony called it iLink) output...I assume yours doesn't.
When I had a similar situation, the easiest (e.g., cheapest) thing to do was to use a DV camcorder as a translator, importing into video editing software. My Canon DV had a menu setting that turned its AV outputs into AV inputs. Take the video and audio outputs from the Hi8 and plug them into the DV, then the FW out of the DV connects to your Mac. If I recall, there was a little bit of hassle getting the Import function to work without a DV tape in the Canon, but somehow it worked.
Otherwise, there are various boxes/cards that can convert analog AV to a digital format that you could buy, or send the tapes to a transfer house that has a high-end converter.