Make sure your iMac can actually run Mavericks:
http://support.apple.com/kb/HT5842
Also note that many of us feel Apple's statement that the RAM minimum is 2GB RAM for Mavericks is a bit.....well, conservative. I prefer a minimum of 4GB for Mav.
If you have trouble indentfying your exact iMac version, please do "About this Mac" from your Apple menu and, in the resulting window, click the "More Info..." button. In 10.7 you should get a screen like this:

The line I've indicated with the red arrow is your model as Apple sees it.