Have you added your user to the vboxusers group? To have the addition take effect - logoff before testing.
sudo gpasswd -a $USER vboxusers
Normally only basic USB1.x is available - you will have to install the extension pack and it must match the version of VirtualBox you are using.
This means - that depending on Manjaro branch and upstream virtualbox - you may need to download the extension pack from virtualbox.org
All these factors are described in the guide.
Right-Click the usb icon - in the running virtual machine status bar - lower right corner of window - the you should have the usb devices