If you're starting by clicking an icon. Put the /nologo switch in target.
"C:\Program Files\Autodesk\AutoCAD Civil 3D 2012\acad.exe" /nologo
Just realized I'm referring to the wrong thing. I'm talking about how to turn on the "welcome" dialog box once it's turned off. Sorry!
Start with some of our most frequented solutions or visit the Installation and Licensing Forum to get help installing your software.