If using the new Sketchup Make 2014, follow these instructions:
STL IMPORTER/EXPORTER: In order to export a file from Sketchup for 3D printing, we must be able to export STL files. We need to install a plugin for this. In the top menu, go to “Window.” Click on “Extension Warehouse.” A new window will now open. There will be a menu on the right side of the window. Click on “Sketchup STL.” Click the large red “Install” button on the right side. You will now be prompted to login to your Google account. After logging in, you will have to Click “Install” again. Accept the terms and click “Confirm.” You must now click “Install” for a third time to start the installation. A window will come up and warn you that this extension will have access to the file system on your computer. Click “yes.” Sketchup will now tell you that the Extension has been installed properly and that it is ready to use.
For previous versions of Sketchup:
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.