I had a similar problem but with Hebrew, not Arabic. After doing some testing, and contacting Corel, I learned that Painter 12 does not support right-to-left text entry. Neither does the newer version Painter X3.
I discovered a workaround: Using a non-Unicode font, Hebrew (and presumably Arabic) can be entered directly into Painter, but the letters are added right-to-left. So you'll need to type the word or phrase backwards, starting with the last letter, and ending with the first. (For longer phrases, it helps to write them down on a notepad first.)
Even Unicode fonts work if you enter your text into a different app first, then copy & paste. Unfortunately, Painter reverses the pasted text, so you still need to type the text in backwards.
I was doing this to help a client who uses a Mac, but it will probably work on Windows as well.
I seem to remember learning that some Arabic letters have a different appearance if they appear at the end of a word. This may create an additional obstacle, but hopefully you'll find some method to work around that problem.
Now, having written all that, let me add that this solution is really only acceptable for small bits of text. I certainly wouldn't want to enter an entire paragraph of text backwards! If you regularly need to use a right-to-left language, then Painter is probably not the right app for you.
If I may be allowed to voice a complaint: The Mac provides excellent support for right-to-left text entry, and Apple has done a great job of building in system level support. Developers don't need to re-invent the wheel, they just need to use standard Apple APIs. It's easy for programmers to do, and even cheap & free shareware apps support it. Only apps that bypass the system APIs and susbtitute their own text-entry routines have this problem. (And the only apps that arguably can justify that are word processors.)