To view the asset library, from the dashboard go to 'Modify an existing item'. At the bottom of the page there is a button for the 'Asset Library'.
This brings you to the asset library, where thumbnails of the assets in the library are displayed. By clicking on these it is possible to change the status and view the versions of the asset. It is also possible to see which items the asset has been used in.
To add a new asset, click 'Create new asset'. This brings you to the asset creation form. The asset ID is pre-filled in the number field, and for most purposes you will not need to change this.
There is a field in the form to add tags to the asset, so that future searching for the asset is made easier. This is a space separated list.