It's actually not a difficult fix. The problem is that the Civil 3D Add-ins tab is somehow not properly configured. The solution is to simply add a new ribbon tab to your main CUI (must be the main CUI and not the enterprise). Either copy the ribbon tab from a proper ACAD.cuix or just create one with the name Plug-ins with an alias of ID_ADDINSTAB (see attached image). Now add that tab to your desired workspaces. I don't think you can simply rename the Civil 3D Add-ins tab; I recall having mixed results doing that.
The not so easy part of the solution is to get the plug-ins that you have already installed sorted out. This can be accomplished by one of two ways. Either manually editing the main CUI to remove the plug-in ribbon tabs (and panels) from all the workspaces (might as well get rid of the useless Civil 3D Add-ins tab too), then adding back in all the ribbon panels which should now be merging as expected. The easier solution is to just unload all the partial plug-ins CUI files, then restart Civil 3D and all of the plug-ins should load back up merged into the 'correct' Plug-ins tab.
This problem was around in 2012 too and I'm pretty sure I filed a report about it. It's really a straight forward fix so it's a shame it wasn't sorted out before pushing 2013 out the door.
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