How can we attach a date calender to a custom form column?
When I see the term "Custom" I think of the Enterprise Business Suite (EBS). Are you working with the EBS? If so, the EBS has built-in methods for using a Calendar. I suggest you look at the Oracle Applications Documentation for your version of EBS and look at the Developers Guide in the User Interface, Development, and Personalization section.
If you are NOT working with the EBS, please tell us where you got the Calendar you are trying to use.