Once your book is in KDP Select, click the box next to the title of your book and then click on the "actions" button at the top. When it drops down, there's a link to the page for managing promotions at the bottom.
I can't click on it since I don't have anything active in KDP Select yet, but I still see it listed.
Aso, if you just click on the title of the enrolled book, it brings you to the 'details' page and up top is an orange box,. The right link, I believe it is Promotions manager, click on that and set it up.
It'll ask the name of promotion, I put my title, although I couldn't figure out what else it's supposedot be/