Are you a "non-partison" voter?
 

Here in California, if you vote by mail and are registered as "decline to state", you should have gotten your notice in the mail that as a "non-partison" voter, have been invited to participate in the primaries for either the Democrats or the Republicans. You simply send the notice back if you want to vote in one of parties primaries and they will send you a ballot as if you were registered in the party you choose. When they issue the mail in ballot, you will send it in with your vote like you normally do.

If you vote at the polls and you are decline to state, you can make your choice, if you want, at the polls when you show up. The poll workers should offer you the choice there and they just give you a ballot for the party you choose.