I selected settings/storage and added a new SCSI controller to the virtual machine. In that controller I added an optical drive with BusLogic. After that I went to the start/settings/control-panel on win98 and added a new hardware. After a few tries and a restart, the win98 recognized the disc and I was able to use it.
I managed to install my program after mounting the ISO file.
It required 256 colors at least. So I installed a display driver from here: https://onedrive.live.com/?authkey=!ALiY0wuD0edfrD0&id=108BFF45BB25F74A!18183&cid=108BFF45BB25F74A According to ESET Smart Security it does not contain a virus. The first and the second restart did not work, but at third restart Win98 managed to boot up. After that I changed the resolution to 1024x768 and set the color to 16 bit.
Done.