Hello there! Sorry we couldnt get back to you faster. I do have a solution however if we use a usb keyboard ( mac or pc) and the enter key works then the issue is with your macbooks built in keyboard if it does not work then the issue is with the OSX . We can reinstall the osx without deleting your user files. Please post back the end result thanks.
No worries but just to clarify i meant u could use any keyboard ( since they are not specific to a certain computer). Below are the steps to reinstall the OSX
1. turn off mac2. Locate/push CMD (command key) and R keys3. Turn on mac while still holding the keys from step 24. Select language
5. Select reinstall OSX.