You can add keywords to single assets, or to multiple assets.
To add keywords to a single asset, go to the Preview Screen.
Click the drop down list in the Keywords section. Select the keyword you would like to add to the file.
Keywords can also be added from an asset's Information Page. Although any user can add keywords to a file, only Admins are able to create the keywords that can be used (in the Settings).
To add keywords to multiple assets, bulk-select the assets you want to add keywords to by clicking in the upper-right corner of the respective assets. Then click the Edit button in the tool bar.
The Edit Content dialog opens. Select the keywords you want to add to the assets from the Keywords dropdown list. When you are finished, click Apply.