If it is not appearing in your purchases list then try unhide it. To unhide an item follow these steps:
1. Open the Mac App Store. 2. Choose Store > Sign In, then enter your Apple ID and password. 3. Choose Store > View My Account. 4. Click View Hidden Purchases from the Account Information page. 5. From the Hidden Purchases page, locate the app you want to unhide. 6. Click the Unhide button.
If it does not appear in the hidden purchases list then you'll have to uninstall your current version of Xcode and reinstall it from App Store.