I bought the morebeer all-grain thanksgiving ale kit, I would like to try to add some canned pumpkin to it, but have never done anything like this before. How much pumpkin to use? Do I put it in the mash or during boil?

Due to the starches in it you will want to add it to the mash. Most of the flavor folks associate with pumpkin comes from the pumpkin pie spices. Pumpkin itself may not be to everyone's liking.

The one time I did pumpkin I started with fresh pumpkin. I cut it up and roasted it in the oven until the pumpkin flesh began to brown, caramelizing some of the natural sugars. I went pretty light on the spices, using fresh grated nutmeg along with some cinnamon, cloves and allspice, all added a minute or so before the end of the boil. The caramelized sugars didn't ferment out and balanced out the spices quite nicely. I've lost the recipe I used or I would post it.

Wayne

+1 to what bug eater specified.

For your spices go sparingly as you can add some if need using a tea bag to the keg.
Most of the recipes I see call for about 0.5 - 1 tsp for a 5 gal batch of each (or in varying amounts) or 1.5 -3 tsp of pumpkin pie spice depending on how pronounced the desired flavor is.