The 3DxSoftware installer usually installs the SketchUp extension automatically. Instances where it may not are if there are multiple SketchUp installations or custom paths. In this case there are steps you can use, to manually install the SketchUp extension.
The usual path for the SketchUp plugins folder is
|Library|Application Support|Google SketchUp 8|SketchUp|plugins
If you have SketchUp 8 installed in a custom path and the installer does not locate it , you will need to create folders in the default path so that the SketchUp extension can be installed then copy it to your path.
First, try running the 3DxMacWare installer with SketchUp closed then open SketchUp.You should see the 3DMouseNews window open . You should also see 3DxSketchUp listed under Windows-Extensions.If you do not , you will need to manually copy the extension.
You can follow these steps:
1) Create a folder called /Library/Application Support/Google SketchUp 8/SketchUp/plugins ;
2) Run the 3DxMacWare installer;
3) Check that 3DxSketchUp.bundle.plugin and 3DxSketchUp.rb were copied to /Library/Application Support/Google SketchUp 8/SketchUp/plugins . If you have a different path , these are the 2 files you need to copy from this location to your SkecthUp plugins folder.
4) Start SketchUp8 . The 3DMouseNews window will open if the Sketchup 8 extension is installed.