Not sure but I can RMB on either the part in the model and or browser window and select "Ground" and the part grounds on my system.
It's difficult to figure out what the problem is and the "SkeletonBase.ipt" is missing from the ZIP file so we can't replicate the problem.
Any chance of a screenshot showing the model and the browser perhaps? Highlight the component that won't ground.
Real easy answer, the part you are trying to ground is in a sub assembly one level down which is why you don't have access to it. If you double click on the "Frame.iam" and open the sub assembly you can then ground the part. At the upper level all you could ground would be the FRAME.IAM file and not parts inside of it.