By default, Sketcup Make does not support STL model files which are often used for 3D print slicing program. To have STL import and export functions, you needs thrid party STL plugins. You can download them stl_plugins.zip. It is a zip file. Here we propose a simple step to install them.

For Windows:

Unzip it into  YOUR_USER_HOME_FOLDER/Application Data/SketchUp/SketchUp 2016/SketchUp/Plugins

For OSX:

Unzip it into ~/Library/Application Support/Sketcup 2016/SketchUp/Plugins

Check if they are enable:

open Windows -> Preferences->Extension to check the STL plugins to enable.