How to connect to a remote Oracle server from Windows machine using PL/SQL Developer?
You need to have SQL*Net installed in order to connect PL/SQL Dev to Oracle. SQL*Net is installed by default with most Oracle DB products (Oracle client for example). Since PL/SQL dev is a Windows app, you can see what Oracle products you have installed by looking into the registry (HKEY_LOCAL_MACHINE\SOFTWARE\Oracle). Also in PL/SQL dev settings (Tools/Preferences/Connection) you will have a list of all Oracle Homes (which allows you to specify which one you want to use if you have multiple homes).

If you already have an Oracle Home, the TNSNAMES.ora file is by default in the directory %ORACLE_HOME%\network\admin. You can set the registry key TNS_ADMIN if you want to specify another directory.