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.