Look in the Plugins folder and you should see a cabwriter folder and a cabwriter_loader.rb file. Make sure it is there.
If that those two items are in the Plugins folder make sure you are looking at the correct Plugins folder:
1. Open SketchUp and the Window > Ruby Console.
2. TYPE this EXACTLY as shown in the lower field of the Ruby Console and press Enter:
3. Look in the upper field and you will see the location of your Plugins folder. Go there and confirm that this is where your CabWriter file and folder is. If not re-install.
If you still have problems let me know and perhaps we can set up a telephone call or and internet screen session.