That is an excellent shotgun you have there. It is 12 gauge and loaded properly will knock a turkey flat at 45 yards if you can aim it... I am not sure if manuals can still be downloaded on the White Rifle web site. Worse comes to worse, contact Doc White and I am sure he could tell you loads, and tips to getting the most out of the gun.
A number of White Rifle and Shotgun owners have a forum of their own where they meet and answer questions and just have a good time talking about their rifles. I am sure you would find someone over there with information on loads, etc... If you would like information on their forum, drop me an IM.
Doc's site is ...
http://www.whitemuzzleloading.com/prod04.htm