Here's some info that may help:
Setting up USB Options
End users wishing to use the USB options will need to adjust the settings under the program settings screen. With VirtualBox, each operating system is listed in the left pane of the program. With the operating system highlighted, settings can be adjusted by clicking on the settings (geared icon) above the selected operating system.
In order to adjust the aforesaid settings; the settings options will appear after clicking on the geared icon. Click on the USB Settings. The right pane will show the USB screen. Check the box to enable the USB Controller. You can now click the empty filter icon (USB with Blue circle) to create a USB that accepts any USB device. The USB icon with a green plus sign can add devices already configured on the host operating system. By selecting these devices, USB will now be ready to accept devices on the virtual operating system once it is started.
Read more: http://www.brighthub.com/computing/windows-platform/articles/28576.aspx
Also, on the VirtualBox forums themselves, it looks like you're not the only one with 64-bit Host USB problems:
That thread is dated Apr. 2010 so it's current, and they've got a few 'solutions' listed there they may help you out.