Open a SQL Server Compact database in SSMS...

...by picking "Connect Object Explorer..." from the File menu and then setting the "Server type:" option to "SQL Server Compact" in the dialog box which appears. Put the route to the file in the "Database file:" option's field.

Addendum 7/12/2013 Note that per a comment left at this blog posting this "Does not work in SSMS 2012, only 2008/2005."