With modern punctuation, here's how I'd render that line.
Addiditque, "sermo mihi est ad te." Cui ait, "loquere." Et ille,
And he added, "I have something to say, in your direction." To him, she said, "speak." And he,
(The "in your direction" part is clunky in English; I'm trying to convey that the Latin uses a very emphatic ad tē instead of the more natural tibi "to you". This might be for emphasis, or might be to avoid ambiguity with two datives in the same phrase, or might very well be a literal translation of the Hebrew, since Hebrew also has the sermō mihi est construction. I'm not sure which is the case here.)
If you want a nice English translation, you should probably cut off the et ille at the end, which is literally just "and he: it's the start of a new sentence that will only be finished in the next verse.
Once you've removed that part, though, your English translation seems solid. "I have something to tell you" or "I have something to say to you" would be a bit more true to the text than "let me tell you something", but the meaning behind them is the same, and that's what really matters.
I'll finish with how I'd translate this if I were going for nice English:
Addiditque, "sermo mihi est ad te." Cui ait, "loquere."
[After a moment,] he added, "I have something to tell you." "Go on," she replied.
As mentioned above, I'd leave the et ille for the next part:
Et ille, / "tu," inquit, "nosti, quia meum erat regnum…"
"You know," he said, "that the kingdom was mine…"
P.S. The verb tenses in this section are also kind of a mess, switching between past and present on a whim. This just comes down to style; Latin allows the "historical present" tense when narrating past events, so there's no real difference in meaning.