Follow this link to a nice plugin for SketchUp that enables you to import STL files: https://sites.google.com/site/jimfoltz/sketchup/my-sketchup-plugins/stl-importer/jf_stl_importer.rb?attredirects=0&d=1.
1. Install the plugin, using the installation instruction provided on the site. (Hint: save it in the plugins folder in SketchUp)
2. After you have installed the plugin, reopen SketchUp.
3. From the file drop down menu, select “Import”
4. A window will pop up asking you to select the file to open. From the “Files of type” drop down menu select “STL Importer (*.STL) by Jim Foltz. Next, select your file name and click “Open.”
5. Congratulations! You have just imported an STL file into SketchUp. If possible, remember to donate to Jim Foltz for providing the plugin.