I looked at
this page but couldn't find any data that suggested that the Axis CodeGen has an Eclipse Update Site associated with it. I've had pretty good luck getting Eclipse plugins to install via the built-in software updates mechanism. Although it sounds like you've done exactly what they instruct you to do, I think this unpack and copy approach may be less reliable.
Also, the Axis page seems to reference Eclipse 3.1, while I'm assuming most people are currently running the 3.4 version. I wonder if there's an issue with the base Eclipse version here?
Peace,
Rick