You can open SQL Compact 4.0 Databases from Visual Studio 2012 directly, by going to
- View ->
- Server Explorer ->
- Data Connections ->
- Add Connection...
- Change... (Data Source:)
- Microsoft SQL Server Compact 4.0
- Browse...
and following the instructions there.
If you're okay with them being upgraded to 4.0, you can open older versions of SQL Compact Databases also - handy if you just want to have a look at some tables, etc for stuff like Windows Phone local database development.
(note I'm not sure if this requires a specific SKU of VS2012, if it helps I'm running Premium)