Open a terminal window and enter
You should get something similar to this:
users lp floppy dialout audio video cdrom plugdev power usbfs netdev scanner vboxusers
If you do not see lp
(if you did not already do that) and, possible scanner
(if you have a printer that is also a scanner), add yourself to those groups either by directly editing /etc/group
(as root); the entries will look like
or with (as root)
usermod -a -G lp,usbfs,scanner,vboxusers yourlogin
You may need to reboot (maybe not).
When you restart VirtualBox and boot a guest operating system if you right click on the icon that looks like a USB stick you ought to see USB devices display there and you should be able to click the associated box to select the device (like a printer, stick or whatever).
You actually do not need to have the entry in /etc/fstab
-- won't hurt but may not help; KDE's Notification will pop up when you plug in, say, a USB stick, ignore it and see if you can select the device in the guest.
Hope this helps some.