During the “Install the Base System” step, you’ll be offered a menu of devices from which you may install the base system. Here, you need to select your CD-ROM device.
You will be prompted to specify the path to the base2_1.tgz file. If you have official Debian media, the default value should be correct. Otherwise, enter the path where the base system can be found, relative to the media’s mount point. As with the “Install Operating System Kernel and Modules” step, you can either let dbootstrap find the file itself or type in the path at the prompt.