Niacin is a vitamin, only ever seen it in a pill. :) Niacin is the name of one of the B vits. B vits are one of the only vits I recommend taking regularly as they are water soluble so you can't OD and they are involved in so many processes.
Most people take about 1 tsp or 1 tablespoon of flax seeds at a time, and usually settle for once a day, altho more is certainly fine. Our guidelines say to build up to 3 a day. It's better to take a regular sized dose many times a day then one single large dose.
To make ginger tea for the bath, get a large piece of ginger, maybe even two. Cut them up, even grate them for best results. Simmer on low heat (tiny bubbles, not roiling boil) for at least 20 minutes. Strain into bath. :)