How do i add flax seed to muffins ?

Sounds like you can replace some of the flour with ground up flax seeds pretty easily! :

http://www.livestrong.com/article/295137-how-to-use-ground-flaxseed-meal-in-bread/
http://chowhound.chow.com/topics/592902

If you're just adding it for nutritional content, you can usually add up to 2 Tbls ground to the base recipe without changing anything else, or more by using it to replace some of the flour. If they seem dry, just add a little more liquid or oil. If you're trying to use flax as an egg replacer, I've had good luck with mixing 1/4 cup ground flax with 1/2 cup oil, then mixing as usual. The muffins will be denser and not as high as ones with egg, but should taste delicious. I can give you more specific quantity suggestions if you list the recipe. Good luck!