Sorry, I should have quoted a bit more from Berry:
"Grape concentrate is perhaps even better used as an additive in fruit wine musts, to improve their vinosity. (This was the purpose of including 1/2-lb or 1-lb raisins -- dried grapes -- in old-time recipes). Try substituting ¼-pint (140 ml) grape concentrate for ½-lb raisins, and include this much concentrate in every gallon of your wine.”
I assume the concentrate would be added at the same point in the process at which you would otherwise add raisins.
I'm curious also about just which type of "concentrate" Berry is referring to -- Welch's, or the concentrates specifically produced for winemaking (and much more expensive!).
I've frequently added raisins to must, but based on this info I'm more likely to use concentrate from now on.