If you own the engine, first do this http://leadwerks.com/werkspace/index.php?/topic/127-if-you-need-access-to-the-forum/
Then download this http://leadwerks.com/werkspace/index.php?/files/file/107-sdk-installer-and-updater/ and use the key you received by email when you purchased the engine. If you somehow lost your key, then email support with your details, so they can confirm you previously purchased the engine and resend you your key.