You can find the plugin on the SketchUcation site (www.sketchucation.com). You’ll need to register, but that’s free. Click on the PluginStore at the top of the page, then search for the plugin you want. Note that with Curviloft, you also need to install a separate plugin called LibFredo. With the Extension Warehouse function in SketchUp 2017, installiing plugins has never been easier.
Hope this helps.