transit visa for 72 hour in China

Hi, we travell from Manila through Guangzhou to Paris and final to Austria. We have to wait for 16 hours in Guangzhou for next plane, and we would like to go to the town, because of 72 hour free entry. What do I have to do to get this permit? Is it enought to ask about it after arrival to the airport, or do I have to send a form somewhere before travelling?