How do I get my bank account to show up so that I can choose it when making a general journal entry to start my company from my trial balance?
You can’t
Please read the guides on Starting Balances
How do I get the correct amount in then?
Manager does not allow journal entries for bank accounts, you need to use payments and receipts for entries see https://www.manager.io/guides/9820 especially:
In Manager, most transactions are entered in other functional tabs, thereby automating many decisions about account posting and reducing errors. So journal entries are relatively few. Most record transfers between accounts. In fact, no transaction involving the actual receipt or payment of funds by a business can be recorded via a journal entry.