There are several ways to get to Redang Island, but most of them require a bit of planning to make sure you don't miss the boat - literally.
There is a small airport on Redang, but tiny Berjaya Air is the only airline servicing it. There are generally three flights a day from Kuala Lumpur (Subang), increasing to four flights a day during high season, and two daily flights from Singapore's Seletar air field.
Alternatively, you can fly to Kuala Terengganu from Kuala Lumpur using either Malaysia Airlines or low-cost carrier Air Asia. Boats from Kuala Terengganu to Redang depart before noon, so be sure to take a morning flight. The boat trip takes 45 minutes to an hour. Several resorts operate their own boat services, so you are advised to check directly with your accommodation.
You can also reach Kuala Terengganu by bus from either Kuala Lumpur or Singapore, but the problem with this option is that you'll likely arrive into the city after all the boats have left, and have to stay overnight in Kuala Terengganu.