How to claim DA from company

I have retired in a PSU recently. Ours is a Govt. of India organization converted into a PSU in 2008 just like BSNL. At the time of corporatization, our Unions/Association have entered into an tripartiate 12.(iii) settlement with GOI, Unions and CLC. We have been given an option to remain in GOI or opt to the PSU. in the PSU we have an option to take pro-rata Pension for the services we have rendered in the GOI and continue in the PSU or to take combined pension for the entire service (GOI + PSU)after superanuation. The pro-rata pension is from GOI and the Combined pension is from Pension trust managed by the PSU. Upto 12/2016 we were in CDA scales only I have opted for pro-rata pension in 2008 and recently retired. I am getting pension but without any DA component. I came to know that I am eligible for DA also from 2008 onwards. Please let me know the correct rule position. If I am eligible how to claim it and the correct forum to whom I have approach.