This help guide will guide you on how to download the theme file (once you have bough theme) from the download page. Be sure to log in to your themeforest profile before proceeding to this tutorial. Then after go to the download page.
Then goto the theme file and click on download button, a dropdown with few options will appear. Click on the Installable WordPress file only and wait for the download. The file which you will get after download is the theme file which you have to use while uploading in your wordpress dashboard.