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If I have understood, you have a new MacBook and you want to use iWeb with that MacBook. Is this correct?
If that's what you want, you can use Migration Assistant to migrate your files from the iMac to the MacBook, including your applications > http://support.apple.com/kb/HT4889 This will migrate your web info, also, so it's the better option
Copy the iWeb application from you iMac to the Applications folder of your MB. Copy the Domain.site file from your Users/Home/Library/Application Support/iWeb folder of your iMac to the same folder on your MB. However, since you're running Lion on the MB be aware of the following:
In Lion and Mountain Lion the Home/Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
To open your domain file in Lion or Mountain Lion or to switch between multiple domain files Cyclosaurus has provided us with the following script that you can make into an Applescript application with Script Editor. Open Script Editor, copy and paste the script below into Script Editor's window and save as an application.
do shell script "/usr/bin/defaults write com.apple.iWeb iWebDefaultsDocumentPath -boolean no"
tell application "iWeb" to activate
You can download an already compiled version with this link: iWeb Switch Domain.
Just launch the application, find and select the domain file in your Home/Library/Application Support/iWeb folder that you want to open and it will open with iWeb. It modifies the iWeb preference file each time it's launched so one can switch between domain files.
WARNING: iWeb Switch Domain will overwrite an existing Domain.sites2 file if you select to create a new domain in the same folder. So rename your domain files once they've been created to something other than the default name.