Once you picked a theme and opened it, click Install Now, select the Magento Connect version (if you are using Magento 1.5 or newer, select version 2.0, otherwise leave it at 1.0), agree to the license agreement and click on the Get Extension Key
The extension key will be visible. copy it.
Open Magento Admin >System >Magento Connect >Magento Connect Manager. Enter the admin login details and paste the extension key in the extension key install field. Click on Install.
Once the installation is complete return to the Magento Admin and open System >Configuration >Design >Themes.
Enter the theme name in the Default field and click on Save Config.
The front page might not load. The reason could be the columns in the new home page layout.
You can fix this through the Magento Admin >CMS >Pages. You will find two home pages. One is with 2 columns layout and the other has just one. Disable the first one and enable the second.
Done! Now your new homepage wil show correctly.