How to get access to Mom's bank account?

I think I would push back a bit more on the banks request, maybe insist on speaking to a supervisor or someone more versed in this sort of thing. I'm not convinced the Living Trust should make any difference to the POA. POA is your power to act as your mom, speak and sign for her, carry out her wishes as though you were her. So I guess if they were to need this addendum from your mom to set up on-line banking or use her account because it is a Living Trust then it makes sense they need that for her POA but I doubt that's the case. So think of it like this, if you called and said you were your mom, gave all the necessary answers to prove you were her, any of the things she would be able to do with her account on the phone with them are exactly the same things you identifying yourself as her POA should be able to do. You are an extension of her and that is what you sent them they papers prove. Now maybe if you are trying to change the Living Trust account somehow, are you adding your name to the account or taking someone else's off? It might be that the person you were speaking with at the bank is confused because your name is on the Living Trust and you are acting as POA I suppose but again one shouldn't affect the other, they have separate legal standings.

If they still insist on this addendum seems like it's something they should be able to provide then. Have them send or fax you the addendum that needs to be signed and I would assume notarized, fill in the personal info and take it and your mom to a local notary (bank, PO, police station, Town Hall usually have one) along with ID and get her to sign it. You shouldn't have to hire an attorney for this unless I'm misunderstanding what they want you to do. Remember they (the bank) has attorney's on retainer/staff that should be able to easily help everyone figure this out or at least explain it to you if it get's that far, you shouldn't have to hire someone to do that again.

Contact Senior services in your area. There may be a low cost or free legal service for seniors that can help you set this up.
The alternate would be to contact the lawyer that set up the original trusts that can help with the paperwork necessary.