There are several ways of doing this:

• From the menu:
  • go Assets -> Import Package...
• From context menu in the Project View
  • enter context menu -> Import Package...

Then, in both cases, find the standard packages:

• browse to the package you need
  • Windows: C:\Program Files\Unity\Editor\Standard Packages
  • OS X: /Applications/Unity/Standard Packages

You can also navigate to the needed package:

• DbClick the package from Windows Explorer or Finder,
  • (just make sure that you have the target project open or that it was the last opened project)

See how on mac or windows

See also

If you need to Export Packages, take a look at this answer.

Go for more info in the manual

I am having the same issue. I installed the unity trial version and when I go to follow tutorials online to create an environment I noticed that all the trees are missing (and several other terrain assets) I downloaded the terrain assets file from here:

http://unity3d.com/support/resources/assets/terrain-assets

And I am not trying to import it, however when I go to try these two methods:

From the menu: go Assets -> Import Package... From context menu in the Project View enter context menu -> Import Package... Then, in both cases, find the standard packages:

My computer bell rings and nothing happens. Do I have to enable a setting to allow for importing assets or something?