Hi Shwetha,
When you say you are not able to add attachments to tickets, are you having a specific problem when you attempt to attach a file to a ticket in Service Desk? Do you get an error of some sort? There are no special steps - basically if you are in a ticket, you go to the attachments tab, and from there you can upload an attachment to the ticket. I am not sure what steps you were trying to perform that were not working, so please let us know if you can clarify for us.
Thanks,
Jon I.