You don't have to enter your PIN for payments of £45/€50 (or equivalent in other currencies) and under, or at unattended car parks and transport terminals. However, you will be prompted to enter your PIN on your fifth consecutive contactless transaction or when the cumulative amount spent reaches €150 (or equivalent in other currencies).
If you have just received your new sync. card, you need to activate Contactless by making a Chip & PIN payment first.