From Mumbai: You can catch a ferry boat from Gateway of India to Mandwa jetty. From Mandwa jetty a bus will take you to Alibaug (bus fare is included in your ferry ticket). M2M ferry doesn't have this bus service. You can go for the other ferries. Once you reach alibaug, you can catch a common auto (Rs.60) directly to the campsite.
From Pune: You can catch a bus directly to Alibaug. From Alibaug you can catch a rickshaw or auto directly to the campsite.
Note: Ferry will close and won't operate after 25th May. Please plan your travel accordingly.