How important is it to add keywords during the set up of new products. Many of my existing products do not have any keywords and I'm wondering if it makes any difference, and if so, how?

It does make a difference, but don't put broad based keywords on the products, and don't stuff the product with 20 keywords. Maybe try to put 3 or 4 different 3 keyword phrased keywords on the product. This isn't going to help you overnight, so realized that you are going to need to give it some time for the SEs to pick them up, for them to show up somewhere in the SERPs, then for people to click on them, and have you advance in the SERPs. Even though it may take some time they are worth it.

Amen to that !!!!!!!!!!

Thanks all. I better get to work!

The title should have all the keywords, because that also helps your SEO.