Select the listings you want to include in bulk and click on edit. Make up a custom label (SKU) and add it to all the listing in bulk. Then use that SKU instead of item numbers. Just have to enter on SKU once and it will pick up all the listings that you have added it to.
Also, what they don't tell you, it only works on multi-item listings, not single item listings.
Also, if you want to add items after you start the promo, you can just add that same SKU to the listing and it will be included. I imagine that if you want to remove a listing from the promo, you can just delete the SKU and it will no longer be included.