It hardly matters which one you use. If you are talking about a conversation you had yesterday, you would use the past tense, "meant." But if you are in the middle of the conversation right now, then probably "mean" is better. Still, both are fine.
Either could be used, but 'meant' focusses on a particular thing said. "When I saw your dress, I said, 'Take it back'; what I meant was that it doesn't suit you."
As idialegre says, in the midst of a present conversation, you might try to clarify what you mean, and it's not to do with wording.
A: "When I mentioned the Ukrainians, you said, 'They can go jump off a cliff.' Do you mean you're fine if there's a war, there?"
B: "No, I don't mean that. I simply mean that I have no interest in that part of the world."