I want to share this with my employer as I think it makes me a more valuable employee
Unless you'll be able to directly use the things you're learning throughout the program in your day-to-day tasks, it doesn't make you more valuable. By and large, an unfinished education has no value to someone who is already employed in that sector. Even if you finished the MBA, odds are it won't make you more valuable: they hired you without having one so it's obviously not a requirement for the role. The two major exceptions to this are when promotions require certain degrees (rare these days) or in consulting where a degree means that a company can charge more for you (and your salary should also go up after getting one).
As for telling your employer: you should give them a heads-up if it will impact your work, for instance by limiting the amount of overtime you can put in. If overtime is a regular thing in your current position or if the flexibility to occasionally stay late is a requirement of your job then you have a potential conflict. Only you can judge whether that is the case and if so, whether you should disclose this based on how your company treats their employees. If you fear they might push you out or otherwise punish you if you were less available then it's in your best interests not to disclose, even though they'll probably eventually figure it out as your schedule starts to conflict with your work.
In most cases and with most reasonable employers it would be fine to bring up that you're enrolling in an MBA program as soon as you've made the decision or at any point during the program, for instance when you're leaving early. Reasonable managers will expect you to warn them of your new schedule so they can plan accordingly and should be supportive.
