Set the Image PRO EDID to 720x480 and it should pop up in display prefs.

However - you should really see what resolution your LED Wall processor will accept...
- Ditch the image pro and connect directy to it at an acceptable native resolution (1024x768 / 800x600 or simlar)
- then use a mix window in catalyst at 720x480 and run your content 1:1
- ensure that your LED processor is set to 1:1 scaling too

Image Pros are so over-used... when simply figuring out the best native resolutions works much better. Theres no point in scaling up, then back down

(The exception to this if if you have to convert to SDI or similar)

let me know if you need more help

S