There are a few ways to add or assign files and assets to an album. You can even add an asset to multiple albums. 

The easiest way to add files to albums is to drag and drop the assets to the desired album. You can do this for multiple albums. To select an asset, be sure to hover over the top right corner of the desired asset and click on the checkmark: Then, drag and drop the asset to the desired album in the Folder Tree on the left-side:

See below for the other ways of adding files to albums. 

How to Individually Add Files to Albums

1) To add files to albums, navigate to the file's Information Page:

2) Find the In These Albums section in the upper right corner. Click on Add Album.

3) A menu will display with the available albums to add the file to. Select the albums and click OK:

How to Bulk Add Files to Albums

1) Select the assets you would like to move or to be added to an album and select the Bulk Editor icon:

2) Scroll down to the "In These Album" section and click "Add Album"

3) Select the Album and be sure to click Save you are done.