Welcome, my name is Rick and I will do my best to help you with your issue.
Yes, a waterproof membrane should be installed where the porch meets the house before the porch deck is installed. The only thing you can do now is to caulk the gap and keep re-caulking when you notice the caulking start to fail. I'd use urethane caulk since it will bond better to the concrete and the wood. Be sure your surfaces are clean before you re-caulk