API Version 29 is Winter ’14 and Eclipse versions 3.x are no longer supported. From Force.com IDE - What’s New in Winter ’14:
Version 4.2 "Juno”
Note: The Winter ’14 release no longer supports Eclipse version 3.x. If you rely on Eclipse 3.x, you can use an older version of the Force.com IDE, but older versions will not be updated with the latest metadata enhancements.
So, you can install a previous version if you need to use an Eclipse version 3.x, but you will not have access to API version 29 or you can install (or upgrade) to a newer Eclipse version and have access to API version 29.
Look at the updated installation instructions for the detailed step-by-step installation instructions for the API Version 29 IDE.
My answer in this question describes how to install a previous version of the IDE if you choose to do that (see relevant portions below):
From Eclipse you can follow the normal Force.com IDE Plugin instructions, except you can choose to not Show only the latest versions of available software.
Help | Install New Software...
Click Add.. button to add a new site.
Call It Force.com IDE and give it the value of http://www.adnsandbox.com/tools/ide/install/ and click OK.
Untick the Show only the latest versions of available software
Expand the Force.com IDE node and you'll see that there are the previous versions
Note the Installation URL in step #3 is http://www.adnsandbox.com/tools/ide/install/ and NOT http://media.developerforce.com/force-ide/eclipse42 for older versions of the IDE.