Re: Acceptance letter arrived but can't enroll
Unless you have High School Years that allows you to enroll in university as a teen after graduating from HS early, your sim must be a young adult. Since you got the acceptance letter, then I'm assuming that your sim is a YA. Once you get the letter, click on it in your sim's inventory and read it. Then you should get the enrollment option on the mailbox as @GiardiniDiMarzo mentions or on a computer.
Another thing, having the dog might be a problem since, afaik, I don't think pets are allowed in the University Housing venue type which is used by all the on-campus dorms and houses, so you'll have to go from your home lot. I've never tried moving a pet into that venue type so I might be wrong on that.
Hope this helps.
