Look in /mnt or /media.
If the CD drive is mounted, it should be named something like "cdrom" and be in one of those directories.
To see if it is mounted, you can use the command
to have it in "human-readable format" (in Bytes).
It will display all mounted devices.
If it's not mounted, try this:
Once it's mounted, you access it like any other directory with cd /media/cdrom for example.
To get help for a command, type: