Preheat oven to 350° F.

Grease and flour a 13 x 9-inch pan.

Combine cake mix, pumpkin, eggs, oil and flax seed in large bowl. Beat with electric mixer at medium speed for 2 minutes, scraping down sides after 1 minute.

Stir in walnuts, if desired.

Pour into prepared pan. Bake 35 to 40 minutes or until a wooden pick inserted into the center comes out clean.

Cool in pan 25 minutes. Invert onto serving platter.

Cool completely and frost with cream cheese frosting. 10 servings.