Harmony Help?

I have so much trouble finding the mezzo or second soprano parts in songs. I'm auditioning for the Snake in Children of Eden tomorrow. We have to prepare whatever vocal part that we sing in the Snake's song "In Pursuit of Excellence". I've been practing for the past few days, and am still having a hard time. I can find the soprano and alto parts just fine, but that middle part is so hard for me. But does anyone have any tips that would help me find? I don't even know what kind of advice I'm looking for, but just anything that might help? Thanks.