Hi 8 is an analog format, like VHS. You need to convert it to a digital format before iMovie can handle it.
Some Digital 8 cameras can read Hi8 tapes and convert the signal to DV. In this case, all you need is a Firewire Cable to connect to your Mac. Get a 4 pin to 9 pin cable for recent Macs with Firewire 800.
If you no not have a camera capable of converting the tape to DV, there are several third party devices that can do the conversion.
The El Gato EyeTV Hybrid does a good job, and attaches to your Mac via USB. If you do not have a Firewire Port, this is your only option.
The Grass Valley Company (formerly Canopus) makes Digital Analog Converters (DACs) that can convert analog to DV. Many people seem to like these. These connect via FireWire.
I have very recently used the kit Canopus Analog converter, as advised bt AppleMan1958 to transfer hours of my Hi8 video to my G5 Mac ( OS x .5.8 )
It worked very well, but occasionally froze whilst importing, but I got used to it, so I imported only small sections at a time, about 10 m-15 mins, I got there in the end.
Now I'm on HD with a new Mac pentel proc, and still I have not sorted precisely how to get digital into my Mac, AppleMan 1958 had given me advice, which I am working on.
If you need further info on the Canopus, contact through this forum.