The Oracle Client you are using is not registered (in the Oracle registry), and therefore PL/SQL Developer cannot find it. You can either reinstall it, or start PL/SQL Developer without connecting, go to Tools > Preferences > Oracle / Connection, enter D:\oracle\product\10.2.0\client_1\bin\oci.dll in the "OCI Library" field, and restart PL/SQL Developer.


Marco Kalter
Allround Automations