Hello,
1.
You should simply add the code into the field Fixer API key, without the URI prefix.
Then, the exchange rate will be available, once the cron job reloads the rates.
I strongly recommend that you renew your key to something else. Otherwise anyone can use it, now that you pasted it on this forum.
Since there is a limit on how much you can use the key per month, you do not want anyone else using your key.
2.
There is a coin selector on the checkout page once you select “Bitcoin and Altcoin Wallets” as payment method. The dropdown lists all the coins that have enough balance to pay for the cart, so make sure that there is enough balance first.
Please do let me know if you face any more problems.
kind regards