According to this
it should be installed under /usr/bin. I don't know how yum installed clamav, but you can try to d/l and install the package from the link above after removing the one installed with yum.
Note also that you should mount the partition before scanning for viruses. If your pendrive /dev/sdb1mounts on /media/usb, you should use:
clamscan -r /media/usb