You have to move the domain file from your user account on the iMac to your Macbook.
iWeb sucks for doing any real website design. It ferrets all your website resources for every website you make into a single "domain" file, so exporting to use with other software is nigh impossible, and sharing a website file between multiple accounts or machines can be a real bitch.
The file is in /Users/YourUsername/Library/Application Support/iWeb/, if I recall correctly.