Importing SketchUp 3D Models
Many 3D objects have been created in SketchUp, a program formerly owned by Google and now owned by Trimble. A large number of models made in SketchUp are available in the 3D Warehouse. FrameForge for Macintosh no longer (as of version 4.0) directly imports SketchUp files, but don't despair: FrameForge does import models in the Collada (.dae) format, and when you download a model from the 3D Warehouse, there is usually an option to download it in Collada format.
When looking for models on the 3D Warehouse, you may find many examples of the kind of object you're looking for. These are often amateur creations, so quality varies. When you look at the model information, pay particular attention to file size and polygon count. Larger and more complex models may slow down FrameForge, so it's a good idea to pick the simplest model that will do the job.
If you're making your own models using SketchUp Make 2017, you can use the menu command File > Export > 3D Model, and choose to export as a Collada file.
If you are using the web-based SketchUp for Web, you won't have a direct way to save the model in Collada format, but there is an indirect way. Go to the Open Model/Preferences menu and click 3D Warehouse to upload your model to the 3D Warehouse. When you do so, you can select an option to mark the model "private", so the other users won't see it. After a short delay, you should be able to find your model under your account in the 3D Warehouse, and download your model in Collada format.