I think you should just go for it and tell her. Be ready to answer her questions, I'm sure she'll have some. Give her honest and clear answers. I'm sure she'll respect you for telling her and if she is a nice girl as you describe, she will listen and I'm sure be thankful for your honestly. I think you should pick your time, wait until you'll be uninterrupted and without a time limit. This will allow for you two to have the conversation you need without distraction and you will have a good opportunity to explain to her about M.S and about your M.S. Best of luck :)

I agree with what's said above, if you leave it a while and then tell her it'll be quite a shock I think, whereas if you just tell her upfront then you both know the situation and can get on with being a couple without you worrying about how to bring it up and keeping it secret etc... it's not something to be ashamed of and if she's as nice as you think then I'm sure she'll listen, just make sure you explain thoroughly as if she has questions still she may be inclined to start googling and come up with all sorts of specious scary stuff! Good luck sir, you'll be fine!