Are you trying to play it in DOS or Windows.
Amber is not a DOS game.
If you are trying to play it in Windows, then it sounds like you may have tried to use a CD emulation program like Daemon-Tools or Virtual CD at some time in the past, and that there is confusion between the virtual CD drive and the physical drive letters.
The following doesn't matter for Amber, since it is not a DOS game, but DOS doesn't automatically detect your CD. You need to configure MSCDEX in autoexec.bat and add a line for the DOS CD driver in config.sys.
If you check
http://www.gameboomers.com/Members/DOS.htmand highlight the line for
My CD/DVD drive isn't recognized in DOS
you can see an example.