Asked this question today at the forums there.
IIRC you add the inventory in each location. For example, if you have 3 lbs of 20L, you go to the grains section and double click and add the inventory in the appropriate spot.
As for exporting and importing, while in the grains, hops, or whatever section, select export. Find a place where you want the file and select either the individual or all the items you want to export. (just like recipies) Once exported, move the file to the new PC and open beersmith. Select "Import and merge" and find the file. It will automatically update the list with new information and will not duplicate.
Primary - Apfelwine
Secondary - Niagara white wine
Kegged - AH Special Holiday Ale, AH Cream Ale, AH Honey Wheat
Bottled - Continental Pilsner, Island Mist Mango Citrus, Island Mist Wildberry Shiraz
Up next - AH Foster's clone, Labatt's clone, Hard Apple Cider,
The Independence Street Brew House