Unity natively imports Cheetah3D files. To get started, simply place your .jas file in your project’s Assets folder. When you switch back into Unity, the scene is imported automatically and will show up in the Project View.
To see your model in Unity, simply drag it from the Project View into the Scene View.
If you modify your .jas file, Unity will automatically update whenever you save.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.