Sega Mega CD PRO CDX cart probs

Bought myself a Pal Sega MegaCD mainly to play Snatcher. Finally got a USA version off Ebay along with Shadow Of The Beast and picked up a second hand PRO CDX cart as made by Datel without instructions. I just assumed that it would be a case of plugging in the cart, popping in the game and away we go but neither boot up. I know both CDs work fine as I used them in GENS but alas not with the CDX cart. I know that the CDX isn't 100% compatible. Have I bought two non-compatible games or is there some button pressing required when using the CDX? I suppose I could rip the games and convert them if all fails. Any help muchly appreciated.