Some games can be tricked with mounted images but others still need the CD in the drive.
The simpliest way to create an image is this:
1.
Insert the CD/DVD
2.
Open the program Disk Utility, located at Macintosh HD > Applications > Utilities
3.
In the menu bar, select File > New > Image from Volume...
4.
Go through the save dialog, the image gets created. After that, eject the CD/DVD.
5.
Mount the image (double click) and start the installed game. Hope for the best.
There are a number of other programs that can create images like Roxio Toast and Carbon Copy Cloner, however Disk Utility does the job right in most cases.