I borrowed this page from here.
If you own this please let me know and I'll get rid of it. I only hosted it myself since the original seems to have disappeared.
Please enter the number of row (vertical down one side) and column (across top) LEDs Enter 0 for a single screen or 1 if you want this for your second screen etc. Then Select the starting position for your LEDs on the screen.
The generate button will produce the LED array declaration which can be pasted into the Processing code as described in the article
Screen is used for the initial screen declaration but not the main LED array - assuming you just set up one display entry.
This is for one screen, if you are using multiple screens you will need to adjust by using the screen numbers in the LED array.