What if you short the flipper cabinet switch terminals at the flipper board? If the LED doesn't light then, seems like a definite board problem if the power to the board is good.
FWIW - I've had issues with this product (Rottendog DE flipper board) not working right out of the box, but Jim has always been good to me w/ replacing when issues like this arise. I don't see his schematics on his website anymore (rottendog.us), so can't provide a whole lot more insight. Although you should probably double/triple check your dip switch settings and fuses on the flipper board before reaching out to him, if you decide to do that.
If it were me, I'd look for an OEM flipper board on eBay or Pinside and give that a go
ebay.com link: Data East Pinball 3 Flipper Board 520 5076 00 Guaranteed Works 100