Usually when you sign in, you'll be redirected to the Resolution Center, there you should find a link to add your input for a transaction. If not, the fastest way is always to Call PayPal, which can be found by clicking the Contact Us link at the very bottom of all PayPal pages.
If you are the person who sent the money the above poster is correct. Phone PP and ask them to release the temporary hold. If that fails you can always open a dispute against the held funds so the refund button appears, let the other person refund you, and resend the payment through PP or another payment processor (there are many these days).
