Hello !! I had the same trouble.
Here is the solution:

1) Copy the OCI.DLL file from: C:\app\david.g.smith\product\11.1.0\client_1 to C:\app\david.g.smith\product\11.1.0\client_1\bin
2) Open PL/SQL Developer.
3) Close the connection window
4) Go to Tools -> Preferences
5) Click on Oracle -> Connections
6) Put your OracleHomePath into OracleHome
7) Put your OCI.DLL path on OCI library:
C:\app\david.g.smith\product\11.1.0\client_1\bin\oci.dll
8) Click on Force OCI7 mode on OCI8
9) Click Apply, Click Ok. Restart the PL/SQL Developer and try to connect.

It worked for me.

Good luck!!
grin