Installing the theme can be done two ways. You can upload the theme ZIP file using the built in WordPress theme upload, or copy the files through an FTP client.
Be sure to extract the file "mediacenter.zip" from the ThemeForest download before uploading. Using the ThemeForest ZIP file directly will result in a "Missing Style Sheet" error.
Once the theme is uploaded, activate it from Appearance > Themes by clicking the "Activate" link for "MediaCenter".
After you have activated the theme. You need to install the plugins that are required and recommended to use with the theme.