From within PL/SQL Developer:

Tools > Preferences, Under Oracle > Connection

* Oracle Home… (point to where tnsnames.ora file is, under the Oracle Client Installation)

[text]c:\Users\<username>\oracle\instantclient\network\admin\[/text]

* OCI Library… (point to where the OCI library is placed of the same Oracle Installation)

[text]c:\Users\<username>\oracle\instantclient\bin\OCI.dll[/text]