It's not clear what version of InDesign you have purchased either license for (perpetual license) or subscription. If you have a subscription to Creative Cloud, you can download InDesign CS6. If you have paid for a license to InDesign CS5.5, you can download it here:
InDesign CS6 will run under Mac OS X 10.6, for example. InDesign CC requires 10.7 or later.
5.5 is no longer for sale through Adobe or Authorized Resellers.
But you don't need Yosemite to use Creative Cloud. The current versions will run on Lion and later. (If you are not yet running Lion, and your machine is capable of it, Lion can still be purchased through Apple.)