How do we legally make one daughter the executor of our mother's estate for a small annuity claimant statement?

My mother died May 24, 2009. Since her death, we received a notice of an annuity with Western National Life Insurance Company. I completed Form 409 and, as instructed by the company, designated the beneficiary as "My mother's name" Estate. Now they are asking for proof of executorship. However, her trust does not designate one executor. It names her 3 daughters as the beneficiaries. Is there a form we could fill out and get notarized designating one of us as the executor for purposes of this annuity. We do not know how much is in the annuity, only that it is less that $10,000. They did tell me it is not very much. Can you please send a form or sample letter that all three daughters can sign and notarize designating me as the beneficiary?