1. Click on Attached Libraries node in Object Navigator and then click on + button.
2. Attach Library dialog window will open, then click on the Browse button to locate the .PLL file (extension of PL/SQL library in Oracle Forms).
3. After that click on Attach button to attach the selected library and then it will ask you to Remove Path then click on No button as shown below:
4. The library is now attached to your form.