Holding shift and hitting enter (return) will result in a line break (<br>).
Having said that, if you need to use that to separate items from headers, etc, you should really be updating the stylesheet with larger margins instead. It is guaranteed that clients will eventually forget to do things – it’s best to keep the most amount of control that could break the overall design of the website out of their hands.
Regarding the missing visual editor…
Try going here: Admin > Users > Your Profile
Ensure that the "Disable visual editor" is unticked.