To create or import a BOM click the "New BOM Tab" menu.  You can open as many tabs as you want at once.

The first thing to do is create a "root" item - this item doesn't need to be in the inventory database.  All other items need to be added as children of this item in order for the prices to roll up to a single item. 

Right click anywhere in the whitespace to open the context menu, select Add/Insert blank item here (or you could insert an existing item, create a new item or insert a saved BOM).

Give it a description and set the unit quantity.  After you've added items, changing the unit quantity recalculates the costs all the way down the tree, as we'll see later.  FYI you can create multiple root items if you want to store multiple assemblies in the same file.  But they won't roll up into a single cost.

You can now add items under this node. From the context menu, selecting "insert existing item here" opens a window with the entire inventory to select from.

You can filter items by typing in the Filter box.  You can filter on a selection of fields.  Here we see all items with the word "crystal" in any of the text fields.  Select an item and click OK or double-click on an item.

As you add items you can see the stock usages, unit costs and cost rollups at each level.

You can adjust the unit quantities of any item.  Changing one at any level recalculates the entire tree.  BOMcalc will use the correct prices based on the number of items required.

Here we have changed the unit quantity of the root item to 5.  Notice all of the stock quantites and prices change accordingly.

Notice the checkboxes across the top of the tree view.  Ticking these boxes provides different views of your BOM costs with in stock, on order and to order considered.  With none of the boxes ticked, these metrics are not considered and stock shortages, etc. are calculated as if you had no stock, etc.  This gives you a picture of how many of each item and how much it would cost if you had no stock at all.

This time we have also ticked "include on order qty" to see how it affects things.  Notice that now the total cost is blank ($0) now because there's enough stock to cover the production.