DOS needs a driver and other software to read from a CD drive, it doesn't by default, so if you just have a plain DOS disk without a CD driver that might be the problem. Usually the default drive letter for a CD is D:, if you only have one hard drive or partition that the OS can recognize.
The windows 98 CD, if it's an original or has been burned as bootable, will boot to the install or to DOS with CD support if you choose, but you have to make sure the system's boot order is set to boot to the CD before the hard drive or it will disregard the CD.
Do you know how to get into the CMOS setup? If you're lucky it'll tell you what key to press right after you power on the computer, but it's usually Delete or F11, maybe one of the other F keys.
Once you're there, look for 'boot order' or something like that, and set the hard drive to be below the CD drive.