Go to the Jerseyjackpinball.com and under Support are downloads and a manual. The manual has instructions to do updates full install and delta.
You have to do a full install of 6.06, and then the delta 6.61
No idea what you are doing wrong, if you followed directions. Download the update, open unetbootin, use it to put 6.06 on a formatted memory stick with nothing else on it. Go to game, turn it off. Plug in memory stick, turn game on. It should do it's update by itself. When done remove memory stick, turn game off. Wait a minute, turn game on. It should be in there.
It takes a long time to load a full install onto the memory stick, and a long time to put in game. 20 minutes or so isn't uncommon.
So only a few things can go wrong. Corrupted update file, not put on memory stick right, game doesn't like memory stick.
Or one other possibility. If this game is an early prototype game, you'd have to call the factory during business hours to check the serial number, the first prototypes can't be updated. There are some of these around. Distributors or owners didn't want to give them up and kept them.
LTG : )