Hi, thank you very much!
The confusion about the dam is very understandable, and i'll add a button on the dam menu itself for each beaver to have them start working on the dam.
Right now what you need to do is go to the animal menu in the bottom of the screen, there you can tell each beaver what to do. So you can set them to building the dam, stockpile wood or feed the fire.
I can definatly see how this is easy to miss.
I will investigate the bug asap and let you know.