When you create a virtual PC choosing from one of their choices (i.e. DOS, Win95, Win98, Win2K) all it does is optimize the virtual PC for that OS. It doesn’t actually install that OS. You need the original OS software (whether it’s floppy disks or CDs) available to install into the virtual PC.
You can place your original Win98 CD and mount it into the virtual PC and then boot from it. Or, if you have a USB floppy drive, you can mount that in the virtual PC and install DOS from floppies (if you have them).
You cannot run OSX in it as it only supports x86 processors. OSX was designed for the PPC processor platform. Also, OS X requires a Mac ROM file which Apple won’t release to third parties.