It automatically detects the type of .stl file and imports accordingly.
To import a .stl file, go to the File > Import menu, and select STL Importer. Optionally press the Options button to select the units, then select the file to import.
The STL Importer is free software licensed under the GPL.
Download the attachment from below, and copy it to your SketchUp/Plugins folder.
For Windows, this would be located here:
C:\Program Files\Google\Google Sketchup 7\Plugins\
For Mac, this would be located here:
/Library/Application Support/Google SketchUp 7/SketchUp/plugins/
Here are 3 sources of interesting .stl files to try:
[ Download ] STL Importer.