That version of iWeb is almost 10 years old! It won't run on your computer. You'd have to buy an older version of iLife on Amazon.
iWeb is no longer being developed and I wouldn't recommend you start building a website with it. The current version (which hasn't been updated in nearly 5 years) has lots of issues on Mavericks and Yosemite.
If you want something as easy as iWeb but more powerful and modern, try EverWeb.
There are other web development products on the Mac including; RapidWeaver, Sandbox, Flux, Dreamweaver and a few more.
Hope this helps!