Dextrin Malt is also known as Cara-Pils and Cara-Foam. It adds body and head retention with minimal flavor and color (it's usually rated at 1L-3L).
It adds body because it raises the OG without adding fermentable sugars; therefore the FG is higher, too (more residual sugars).
Have you tried the unaltered recipe yet? Since it is a stout, I'm betting there are already other specialty grains. Caramel/crystal malts also add body (along with color and flavor). So, I don't think dextrin is really needed. If you do add it, use no more than a half-pound (maybe even just a quarter-pound).
From a tip from the LHBS owner: add a teaspoon of citric acid to your priming solution to make a creamier carbonation. My first batch was a stout I did this to. I don't have a direct comparison, so can't tell you if it really worked.
Edit, I looked up the kit. It's a sweet stout, so it contains lactose, which is unfermentable milk sugar; this, along with whatever other specialty grains, should give you plenty of body and mouthfeel without dextrin. Dextrin is primarily used in light beers with little or no specialty grains and/or with adjuncts that thin the beer out.
