This issue seems to be a hot topic lately so I thought I would write a quick post on the subject. Have you been experiencing issues when you try to add new part sizes or make other changes in Part Builder? In most cases this is a permissions related issue, so if you are experiencing this, please read on.
First step is to select the Civil 3D icon, right click, and select PROPERTIES. In the resulting dialog, please select the COMPATIBILITY tab and check the RUN THIS PROGRAM AS ADMINISTRATOR, then click OK. After doing this please try launching Civil 3D again, and see if you are able to make and save the edits. You can also do this directly from the icon's right click menu, but that will apply only to that session of Civil 3D.
Even though you may have full administrative rights on your machine, I have still seen this issue occur, especially on Windows 7 machines. Another place to check would be to ensure that you or your users have full control of the C:\ProgramData folder as this is where the part catalog is located if running a typical install. Hope this helps!