I am assuming this is an older iMac.
You can either do two things.
You can backup your data and revert back/do a complete clean install of OS X 10.6 Snow Leopard OR you can run OS X 10.6 Snow Leopard in virtualization software.
If your iMac, originally came with OS X 10.7 Lion, then your only alternative is to run OS X 10.6 Snow Leopard in virtualization mode.
To do this, you will need to purchase OS X 10.6 Snow Leopard Server and a virtualization software of your choice.
There is VMware Fusion, Parallels Desktop or VirtualBox.
Don't take "NO" for an answer!!!
You can either "dual-boot" as the last poster suggests, or as he also suggests, be able to have concurrent access to your CD-ROM and Lion or Mt. Lion
Here is an example of the educational CD-ROM ("Teacher Express") requiring PowerPC running in Snow Leopard Server (with Rosetta) installed into Parallels 8 for use in Lion and Mountain Lion:
[click on image to enlarge]
Apple is now selling Snow Leopard Server for $19.99 + sales tax & shipping costs at 1.800.MYAPPLE (1.800.692.7753) - Apple Part Number: MC588Z/A (telephone orders only).
Parallels offers a 14 day free trial download of Parallels 8 at its website:
Parallels 8 sells for $79 retail and can be found at a discounted price if one searches the Internet.
The OP isn't talking about the book, itself.
The OP is talking about the CD-ROM disc that came with the book.
The CD-ROM works with OS X versions only up to OS X 10.6 Snow Leopard, but the OP's iMac is on OS X 10.7 Lion, now and the OP can't load or install whatever software is on that software disc, now.
I have seen this problem posted frequently.
Texbooks with CD-ROMs written in the early to mid-2000's and of course requiring Rosetta to translate PowerPC.
With schools strapped for funds in the modern era, there is just no financial incentive for the textbook publishers to rewrite the CDs for Intel.
Hence one of the 4 common workarounds to the lack of Rosetta in Lion and Mt. Lion must be attempted.
I purchased the Teacher Express Demo CD-ROM off of Amazon and my earlier screen shot shows that it indeed will run in SLS in Parallels 8; so I suspect all of them will, too!