You just need get the appropriate Extension Key here http://www.magentocommerce.com/magento-connect/ebay-magento-integration-order-importing-and-stock-level-synchronization.html

If your Magento version is less than 1.5, then you should get Key for Magento Connect 1.0.
If it is 1.5 and greater, then you should get Key for Magento Connect 2.0.

Then you should proceed with install via Magento Connect Manager (System→Magento Connect→Magento Connect Manager)