iWeb stores the data in a Domain.sites2 file whose default location is shown here...
Move this file to your new Mac and double click it to launch iWeb.
See this page for how to find the library folder and open a domain file if you are using Lion/Mountain Lion...
Well of course you won't find an "import button", because iWeb has no import facility and cannot open html and css files.
You need to move your domain.sites file to your new Mac and double click on this and iWeb 09 will open it. Your domain.sites file is your iWeb data file where all your iWeb info is stored. It can be found under your User/Library/Application Support/iWeb/domain.sites. Remember that it is your User Library and NOT your System Library as iWeb is not there.
iWeb is a one way street and only creates html and css files upon publishing with no import facility.
The reason I change computer is because my old computer died. I have backup but the last Time Machine Backup I have does not have my website. I only have the files that are exported when you do the website (the htmls, etc) and I don´t have the domain.site ..
My site is running but I wanted to update it.. is there another way I can open it in iweb?
NO there isn't and if you don't have the domain.sites file then you will have to re-build your site from scratch in iWeb.
Remember from now on that your exported/published files from iWeb are NO backup as iWeb can do nothing with them.
You will need to re-build from scratch and remember to backup, backup and backup your domain.sites file this time!