Depending on a number of factors you could run into problems when paying at PayPal.
Some common issues are:
I am not getting redirected to PayPal at all
Your browser's PayPal cookies might be messed up; you may try deleting your cookies to fix this.
PayPal is not accepting my credit card
Please double check the data you entered at PayPal's form. If everything is correct, chances are that you actually do own a PayPal account and that your credit card is associated with that PayPal account. If this is the case, then you will only be able to pay with that card by logging into your PayPal account. If this is too much trouble, then just try paying with a different credit card.
I can't find a "Don't have a PayPal account?" option
Unfortunately, PayPal does not allow direct credit card payments in all cases. Sometimes it is because of your location, or sometimes PayPal might believe by checking your browser's cookies that you actually own a PayPal account. You might try deleting your cookies to get around this.