Just a small observation: you keep mentioning conn usr\pass, and this is a typo, right? Cos it should be conn usr/pass. Or is it different on a Unix based OS?
Furthermore, just to be sure: if you use tnsnames, your login string will look different from when you use the login method you started this topic out with.
tnsnames.ora should be in $ORACLE_HOME$\network\admin. That is the Oracle home on the machine from which you are trying to connect, so in your case your PC. If you have multiple oracle_homes and wish to use only one tnsnames.ora, you can set environment variable tns_admin (e.g. set TNS_ADMIN=c:\oracle\tns), and place tnsnames.ora in that directory.
Your original method of logging on (usr/[email protected]:port/servicename) should always work. So far I think you have all the info, except for the port number, which I am sure your DBA will be able to give you. If this method still doesn't work, either the server's IP address is not available from your client, or it is a firewall issue (blocking a certain port), or something else not (directly) related to Oracle or SQL*Plus.
hth! Regards, Remco