Some users report problems when they try to download Mavericks from the App store. This is due to the massive amounts of downloads that are causing problems on the Apple servers. It is free so everyone who wants the new operating system can download it and since there are so many Mac users it simply overwhelms the servers. For you to be able to download Mavericks, you probably have to wait, until the traffic sinks. Be patient is the best tip I can give right now:)If it still does not work, check if Mavericks is compatible with your mac, with other words do your mac meet the requirements?