Here's what I found on kellymom:
Both flaxseed and flaxseed oil are reported safe for nursing moms.
Cracked flaxseed is used as a laxative, and should be ingested only with adequate liquid. When flaxseed is taken as recommended, there are no known side effects. Not for use by persons with bowel obstruction. Flaxseed is not absorbed systemically, and therefore poses little threat to the nursling during breastfeeding. More here.
Flaxseed oil is rich in omega-3 essential fatty acids. It's a dietary source of LNA (alpha-linolenic acid). The Food and Agricultural Organization recommends that at least 6% of the calories of a lactating woman's diet be from the essential fatty acids (linolenic acid [LA] and LNA). Both LA and LNA play a very important role in brain development.
I've heard from one source that flaxseed oil can increase milk supply - I have no idea if this is true. One midwife suggests taking an essential fatty acid supplement (flaxseed oil is rich in omega-3 essential fatty acids) to aid milk supply, because the fatty acids help us to make the hormones we need for breastfeeding.
I took it too.
Here's the link: http://www.kellymom.com/herbal/ref/herbs_f.html