First things first, don't panic. These things happen. 

Before you arrive on the project you will have been given the Volunteer Project Leaders contact details. As soon as you know you are going to arrive late to the meeting point, or perhaps will have problems getting there, contact them and they will be able to help you. 

If you have to get public transport from the airport to the meeting point, it's worth making a note of the different departure times of your bus/train before you leave home. This means if you do arrive at the airport late, you will be able to tell your Leader what time you will be arriving.