Question: But Machir and Ya'ir, sons of Menasheh were also born in the lifetime of Ya'akov and entered Eretz Yisrael. Moreover, they were descendents of Yosef. So why did Moshe not ask them?

Answer: The secret of the redemption was handed to Serach, as is mentioned in Pirkri de'Rebbi Eliezer (Perel 48). That is why when Moshe arrived, the elders of Yisrael went to her ...

Reason: bearing in mind that they had already slipped up through the B'nei Efrayim, who left Egypt thirty years before the appointed time.

Answer: So they now consulted her. And when she asked them what expression the savior had used, and they answered "Pakod Pakadti", she replie 'If so, he is the one!'

Conclusion: And that explains why Moshe now went to her as well, From the Tosfos of Tosfos' Rebbe.

Clarification: The Aruch, citing Rabeinu Chananel, says that according to some he was an angel, whilst others maintain that it is the name of a Chacham.