Here is an urgent message from the Painter team at Corel:
Apple announced that the new operating system, Mavericks was ready for FREE download on Tuesday October 22nd. Based on historical release practices at Apple, our team was not expecting this until late November.
As of today, October 23rd, Painter X3 does not completely support Mavericks. To maintain your current user experience with Painter, we would recommend you NOT upgrade to Mavericks at this time.
We are looking at the overall user experience of the final version of Mavericks and we are assessing any required code changes we need to make.
Later this week we will communicate when we expect to have full support for Mavericks. Thank you in advance for your consideration.
Andy and the Painter Team
You can read the official message at the Painter Factory.
In plain English...this means that if you download and install Mavericks (OSX 10.9) you're going to have problems with Painter X3 and probably all prior versions. My suggestion is to wait until Corel has had a chance to work with the new code.