You need to mount the usb first "assuming your on a dvd, vmware, or hard drive install".
First you need to find out what your device is, easy way to do this is to load up gparted, open a console window type gparted and look for the drive / partition that matches your usb. Once you find it take note of label i.e. /dev/*
Lets say for this its /dev/sdb1 we now need to mount it, there is already a folder for usb but you should understand that you can mount a drive to any folder so lets make one. Open another console window and do this.
Now lets mount it
mount /dev/sdb1 /mnt/usb2
Pretty simple no ? .. lets just check that its mounted
And now just feed your application the path to your wordlist like so.
Enjoy, but im going to say this in closing, if you cant even figure some of the most basic linux operations, you should really consider using another distro for a few weeks first.