There two ways to add products to a linesheet:
1) In the product gallery
2) In the linesheet itself
In the Product Gallery
When you are in the product gallery you can select one or more products using the checkboxes. You will see the "Actions" toolbar appear at the top with an action bar on the right.
Select "Add to Linesheet" from this menu.
In the Linesheet Itself
When you are in a linesheet, you will see a blue plus sign on the bottom left of the screen.
After selecting this, you will see a pop up on the bottom of the screen with all your products. You can drag and drop products onto the linesheet. You can also use the search bar to search for the item you would like to add and then drag the image onto your linesheet.