Welcome to our forum.
You do have rights, it doesn't matter if you're a secondary or not, don't be afraid to stand up for yourself and state your needs.
Like km34 said, what are your partner's reasons for not wanting you to date this new person? Are they valid reasons? Do you think they're valid reasons? What kinds of discussions about the problem have you had, and what kinds of things come up?
I hope you can get some of the clarity you're looking for here on Polyamory.com.
Love means never having to say, "Put down that meat cleaver!"