virtualbox.org

vbox v4.1.4
iMAC 10.6

The iMAC keyboard does not have in Insert key. (What idiot thought that was clever?) As a result there is no way to toggle insert/overtype mode in the VMs.

I have seen suggestions like
1. Use the Help key. -- There is no Help key. This is the "full size" keyboard with a keypad.
2. Use the Fn key + <something else>, usually keypad 0 or keypad /. -- All I get is 0 or /; the Fn changed nothing.

Can anyone suggest how to get the Insert key keycode into a VM using the iMAC keyboard?

You can't 'emultate' a key that isn't on your keyboard. If you want to have that key, then get a PC keyboard to replace your fancy iMac one. If you don't want to do that, then you either look for an Apple keyboard that does have one, live with it, or find the keycode that is behind the insert key and use 'VBoxManage controlvm keyboardputscancode' to send a simulated insert. Last resort is to use the onscreen keyboard of the Guest OS.
You do know that some applications have a toggle for it somewhere, like Office has it in the status bar, where you double click on it to change mode?