I'm not really sure what is happening. The steps should be:
- Download the main zip file from Themeforest to your computer
- Un-Zip this folder so that you get a folder with Documentation, PSD, Version.rtf, License, plugins and the abundance.zip
- Now you can try using your Wordpress installation to upload the abundance.zip folder so that wordpress does the theme installation
If that does not work, you will need to login to your hosting server via FTP. You can get directions for this from your host. Go to the wp-content/themes folder and you should see the other themes that come automatically with wordpress.
You now have to upload the unzipped abundance folder into this themes folder on your server. Then you can use the wordpress backend to activate the theme like normal.