If you just connect the FBV Express MKII to a USB port on your computer and run Line 6 Monkey, does Monkey detect and ID the FBV Express MKII? If "yes", continue on to:
Connect your Spider IV 120 to the FBV Express MKII. Keep the Master Volume to "off" and power on. If you select "Change Device" in Monkey, does it show the Spider IV Amp as an option? If "yes", try a re-flash.
If Monkey doesn't see the amp, it'll need to be looked at by a repair tech. If you're still within the 1st year of purchase (based on the date of a new product purchase receipt), an ASC tech can help out: http://line6.com/service_centers/