We're using Land Develpment Desktop and Civil 3D here (2008 & 2009 depending on how obstinate the draftsman is) and I'm having issues adding in custom sheet sizes.
Most machines will allow me to use the plottermanager command then add in the sheet size. At that point, I'll get errors saving it (both the pmp and pc3 files) yet it will save the sheet size (one machine won't let me do this at all). I figure this is probably permissions on the server.
The real question is: If one makes changes on the pmp/pc3 files, which are stored on the server and referenced by all the autocad installations, then why doesn't the custom sheet size display on all machines instead of only the one where it was created?