Win98 does not have the ability to automatically recognize most flash drives (like XP and Vista does). This is because Win98 does not inherently have a catalog of drivers for that type of device. So, the two things I can recommend are that you first go directly to the manufacturer's website and see if they have a driver that is compatible with win 98 and, second, if they don't, you can try this website which claims to have a few "generic" driver downloads that may work for some usb flash drives (see the download links near the bottom of the page):
Windows 98 USB Mass Storage Device Drivers