Simple answer… you can’t! :o)
You can open up to SQL CE 3.5 SP1 in 2008 Management Studio, but CE 4.0 is not supported. Have a quick read of this stackoverflow.com article for more info.
Can you open SQL CE 4.0 files in Visual Studio 2010? Yes you can.
But the problem I’ve come across is that when I want to work in code and write say a POCO class (Plain Old C# Class) I can’t. While the SQL Table property sheet is open so I can see the column data types – it’s Modal, so I can’t easily jump back and forth updating my code while reading the types. Not a major problem really, just an annoyance.
SQL Server Compact Toolbox Stand Alone. You can download it here on codeplex.com. It is a great little tool that you can run – no installation – and connect to your SQL CE 4.0 file. So now I can have the toolbox open on one monitor and happily code away in VS 2010 in the other.
Life is good again.
ps: For all the information you could need on SQL CE, check out ErikEJ’s excellent blog, ironically named Everything SQL Server Compact.