VS 2010 - How To Insert New Row Into SQL Server Table

I'm trying to insert a row into a SQL Server table.

Using this...
cmd.CommandText = "INSERT INTO MyTable (firstname, lastname) VALUES (@firstname, @lastname)"
cmd.Parameters.AddWithValue("@firstname", "bob")
cmd.Parameters.AddWithValue("@lastname", "jones")
cmd.ExecuteNonQuery()

I get an error saying:
ERROR [42000] [Microsoft][ODBC SQL Server Driver][SQL Server]Must declare the scalar variable "@firstname".

But if I use this...
cmd.CommandText = "INSERT INTO MyTable (firstname, lastname) VALUES (?, ?)"
cmd.Parameters.AddWithValue("@firstname", "bob")
cmd.Parameters.AddWithValue("@lastname", "jones")
cmd.ExecuteNonQuery()
...it works fine. What do I need to do to make the first example work? I don't understand what the error message is telling me. How do I declare a "scalar variable"?