My HBS recommends maltodextrin to increase body & head retention, when and how much should be added to 5 gallon batch?
Maltodextrin will increase body using between 1/4 and 1lb in a 5 gallon batch. Although equally, using LME/DME instead of sugar with a kit will do a similar job and give you a much maltier beer. Maltodextrin has relatively little taste, so don't expect the beer to be much sweeter.
To increase head formation and retention, and sweeten the beer, you can add 1/4 of wheat malt (if doing a mini-mash) or a light caramel malt, such as carapils.
I mash from the grains and use maltodextrin to improve head retention, as well as a beer heading liquid containing propylene glycol alginate(PGA), which helps to stabilise the head and prevent collapse of the bubbles. The look and condition of my beers has drastically improved since using these additives but I'm not sure which is most reponsible. To get a good head, the label on the maltodextrin packet recommends adding about 10g (= a third of an ounce)to a 22.5 Ltr(= 5 imperial gallon) brew, AFTER fermentation and just before bottling(same time as I add 10ml of the PGA). This is a lot less maltodextrin than the heroic amounts needed to increase body and sweetness, such as 500g or a pound in weight to a similar brew. I would rather avoid a turgid beer laden with maltodextrins to make it heavier and sweeter tasting. Although the maltodextrin is not readily fermented by yeasts, it is rapidly broken down and utilised as sugar in humans. I prefer not to add more than necessary to my sugar intake while drinking beer! I prefer it dry, hoppy and clean tasting.