End to End Encryption is where the Magnetic Stripe Reader (MSR) encrypts the card information at the read head and it is not decrypted until it reaches the processor. This mechanism prevents hacking of the card information between the pin pad and the computer, in the computer memory (virus/Trojan) and over the internet.
(preferred, most secure) Select Manual Entry on the credit card authorization screen and enter the card number and expiry date on the pin pad keypad.
This will then create a token, which can be used for further transactions.
If you want to enter card numbers using a back office computer that does not have an attached pin pad then ask Mercury Payments for a tokenized non-E2E merchant account to be created. You can then set the merchant number for this terminal to that merchant account.