I think that it was a little misunderstanding. In the answer to your question (from "jsorense") you have an EXAMPLE :
connect system/manager@mydb as sysdba
In this command "mydb" is a name of the database. Oracle try to find information about this name in the tnsnames.ora file (TNSNAMES.ORA is located in a directory similar to the following:
<Oracle Home>\Network\Admin )
You have an error becouse Oracle can't find information about "mydb" in tnsnames.ora file.
I have a very bit experience with Oracle personal editions, but as I know, you don't need construction @<database_name>.