Under ERISA, you will need to obtain a Qualified Domestic Relations Order (aka "QDRO"). Call his employer and ask for a sample form. You can start collecting your percentage of the community portion of the pension when you reach retirement age.
If it's a qualified plan, it is community property during the period of the marriage, under the famous Brown case. You will need to consult with an attorney, who will arrange a Qualified Domestic Relations Order (QDRO) drafted for you. This is a detailed order, which instructs the plan how to split the retirements benefits when it is finally distributed (upon his retirement).
A weekly guide with tips and legal advice for each stage of the process.