What
@JCitron said.
If you don`t see Status in the column-title bar of your Content Manager, right-click on one of the existing column titles and use the resulting pop-up menu to add the Status column. It should be already there by default, though.
Left-clicking on a column header sorts the list by that column. Available for download is one of the annotations that can be found in the Status column. The reason for sorting by Status is to group all of the downloadable items in a bunch, instead of having them spread out throughout the list. This makes them easier to select as a bunch.
Do you understand, or do I need to try again?