In SketchUp 2017 the System Preferences box was renamed to SketchUp Preferences and Extensions was removed from it and placed under Window and called Extension Manager.
To install the .rbz files (CabWriter and CutList Bridge DXF) you have downloaded follow these instructions for each .rbz:
Download the .rbz file to a folder where you can easily find it i.e. a folder you know the location of. (Your Downloads folder for example.)
1. We recommend logging into your computer as an admin before installing any Ruby scripts. This will make the installation go more smoothly and ensure that files get installed in the proper places.
2. Select Window > Extension Manager. The Extension Manager dialog box is displayed.
3. Click on the Install red Extension button at the lower left. The Open dialog box is displayed.
4. Locate the Ruby zip file to install (.rbz) and highlight it using the cursor.
5. Click on the Open button.
6. The Extension Manager list will change to include extension you installed (you may have to scroll to see it) and should be Enabled on the right.
I hope this clears things up.