To access the item manager you just have to click on the Items option from the left sidebar menu. Once you've done that, you should see an empty list of items opened up on the right. Having a pre-made list of items allows you to create invoices more easily, because you don't have to manually fill out item information when adding them to the invoices that you create.
List consists of 7 columns. First is the item ID, followed by item number, name, rate, unit, tax and at the end you get options for managing the items (preview, edit and delete). Above the list, on the left you can see a filter for filtering (switching) between currently active items and the ones that were deleted (moved to trash). On the right there's a search tool for quickly finding the items that you need when there's a lot of them in the database. To make it easier for you when managing the list of items, it's possible to switch between the default 10 items per page, and change that to either 25, 50 or 100 items per page. Use controls in the bottom right corner to switch between the pages. To start the creation of a new item click on the Create new item link in the top right corner of the screen.