OKAY... there are a variety of option available, and the one below can be used as a "quickfix" for bettsaj, but ONLY IF PayPal is being provided as a payment option. This is because we are about to alter a GENERAL DEFINE that will appear on the page irrespective of whether paypal is active or not, so only use this method if paypal is active...
Via FTP, port a COPY of: includes/languages/english/checkout_payment.php to your local drive.
Open it for editing and find:
You could add something like:
define('TEXT_SELECT_PAYMENT_METHOD', 'Please select a payment method for this order.');
Then SAVE the file on your local drive.
define('TEXT_SELECT_PAYMENT_METHOD', 'Please select a payment method for this order.<br /><br />PayPal must be used for all credit card or debit card payments and you do not need a PayPal account to do this.<br /><br />PayPal will invite you to open a PayPal account, but this is at your discretion. You can still pay us using a payment card as described.<br /<br />If you do have a PayPal account, you can either choose to pay using your account, or use another credit/debit card.');
Then FTP it to: