Fundraiser organizers have the option to choose between WePay and PayPal as a payment processor for their fundraiser. The payment options available to you will depend on which of these providers has been set up by the organizer.
PayPal will only appear as a payment option if PayPal was chosen as the donation processor for the page. If PayPal does not appear as a payment option, you can choose to pay via credit card through the selected payment processor, or via bank transfer in some cases.
Many fundraisers are also happy to accept donations by mail. If direct donation instructions aren't included in the fundraiser description, you can click the "Contact the Organizer" button on the fundraiser page to see if they can accept donations by check or cash.