2 nd step : u just post ur application in form -I ( copy attached) through registed post, If they are not replied within 30th days then just go to DLC office for complain along with reject.
Any problem pls cal me it's easy to solve the problem
Offer Letter & Proof-of-Joining are 2 different things.
Like Abhijit Barua suggested, use 'payslip' and/or 'Appointment Letter'--the later is usually given AFTER joining.
If you don't have the practice of 'payslips', your Bank Statement [to prove that salary was being deposited into your Bank Account] should suffice.
While using such documents in interviews are understandable, it's quite surprising that one has to go thru this ordeal with a company where one worked.
All the Best.