Hi Scott,
If you open your listing back up, right under the title section you'll see the section for selecting your ebay categories for an auction; if you have an ebay store, below that there will be drop-downs for you to select your store category as well (if you don't have a store, they will not be available to you).
Also, I think that if you don't choose a category, the system won't let you post it...so you may want to go back and check to see that maybe you did pick a category, but perhaps not the one you inteded.
Good luck
Kathleen