Normally you will get your National Insurance credits automatically if you are already claiming Employment and Support Allowances. You can check if you have got credits by calling the National Insurance Enquiries for Individuals Helpline (0845 302 1479) or contacting your local Jobcentre Plus office.
If you have not got the credits and think you are qualified for it, you need to apply to HM Revenue & Customs. For forms and more information, go to www.hmrc.gov.uk/ni/intro/credits.htm#6