You can file a complaint at the nearest SSS office, or preferably, at the SSS branch that has jurisdiction over the employer's records. It would help if you could specify what months or years the employer failed to remit SSS contributions, and if you could bring proof of the same (e.g. pay slips for the period in question).

Pwede ka rin tumawag sa SSS Member Relations Department para mag-inquire sa specific procedure. Nasa web site nila yung number.