Have you followed the guide here – https://support.redfactory.nl/documentation/restaurant-theme/integrating-cart66/
The way it works is you have to set up your products first in the menu card, then add them again in the cart 66 plugin and link them. So once you’ve created all of your products for the menu card, you then go into Cart66>products and start adding them in there. Once they’ve all been added (do one or two first to see how it works) go back into the products you created in the menu card and you’ll see in the top right hand of the editor a drop down has appeared with products you created in Cart66. Select the one which is relevant to this item on the menu card, enter the price and save.