You are able to add Line Items through your settings,

  1. Select Settings cog from the top menu bar.
  2.  Accounts
  3. Items
  4. Select the green + New Item button to add a new Item.

Alternatively you are able to add line items when you create an Invoice

  1. Select CRM from the top menu bar
  2. Select Invoices from the drop-down menu.
  3. On the right hand side of the screen, select the Green + New Invoice button.
  4. Whilst creating the Invoice you will select the item drop-down.
  5. Select New Item from the Menu.