Lots of treats, repetition, and patience. Like any trick, dogs need to be carefully coaxed to begin with. You may need to simply let your dog explore it before trying to get them into it. Once they seem comfortable being around it you can then gently encourage them to climb inside - rewarding each small step so as to create a positive association with it. Positive reinforcement is key with training a dog, so keep telling them they're doing a good job, and eventually, they will get used to being inside a kayak.