How do I start a process instance from a SQLServer table change?

You could create a SQL trigger that invokes a small .NET console app.  The console app should then use K2ROM.  Additionally if this is SQL 2005 I suppose you could try writing a .NET CLR stored procedure that is called from the trigger.  This CLR trigger would use the K2ROM API.

No matter what path you select you will need to invoke the process via the K2ROM .NET assembly/API.  Check the documentation for how to code against K2ROM.