The only way to get to Pulau Rawa is via a ferry from Mersing jetty. The trip from Mersing only takes about 20 minutes by speedboat. The total journey time from Singapore to Pulau Rawa is about 7 hours including the bus ride or 4 hours in a taxi.
If you have taken up a package with a resort beforehand, simply show up at the Rawa Safari office, which is located in the middle of the gated carpark next to the ferry terminal in Mersing. There are always enough people around to show you where it is exactly.
Alternatively you could request for the ferry heading to Tioman to make a stop at Rawa. The ticket will cost you 45RM. When at the ticket counter, get your return ticket with a date and time on it. Ensure you have the ferry operator's number in case the boat doesn't show up on the day of return. However, this only makes sense if you are on a day trip. The resorts on Rawa offer package accomodation only, and the boat transfer is included in the package price.
If you are in Salang on Pulau Tioman, Armila Sports Centre can get you a ticket to Pulau Rawa for 45RM as well.
Resorts on Pulau Rawa recommend that you book directly with them to prevent from getting ripped off or problems with the ferry.