I signed up to present at a conference and didn't tell my advisor about it. How to handle this now?
Explain it just like you explained in your post.

Tell your advisor that you are very interested in attending a particular conference and that you even applied. You realized it was an oversight that you applied without talking to him first, and then you got sidetracked.

Make it clear that you want to go, that you got accepted, that you made a mistake, and you'd appreciate his feedback on all of it. If your advisor tells you to withdraw, then I would probably listen.

It is not common to do such a thing without discussing it with your advisor, often because they are a co-author and footing the bill.