Running bear I hate to say this but it looks like you need to read up on some things. I would start out by reading up on net manager. Even though you've already done so, I would never mess with the tnsnames.ora manually, that thing is just too fragile, one wrong syntax and the whole thing is messed up. Could you imagine trying to edit a tnsnames with about 20 databases in it? Unless you've got some serious experience under your belt leave the thing alone. :)
Anyway, in all honesty you're getting wayyyy to deep into something that is really simple.
Without having a live conversation to guide you, read up on adding a service name.