You need to have a Premier account to accept payment from other people who are using a debit or credit card. If you've sold stuff before and received payment through PayPal , then you've got lucky and the people paying you have had funds in their PayPal accounts - this means PayPal class it as a transfer and you don't need to upgrade.
The fact it's an international transaction has probably been picked up and as such you will need to upgrade. It's not the most difficult process - they will take two small amounts of less than �1 off the card you choose to link to the account to verify it - you get them back so don't worry.
The downside is that they start charging you to receive payments through PP - every time they take any payment under �1500 they charge you 3.4% + 20p. Over �1500 and the percentage rate reduces.
To upgrade, you'll need to log into your account and follow the instructions - I'm not sure exactly where you'll find them as I upgraded ages ago and can't remember.
HTH