Your host has a getting started guide to FTP (http://www.inmotionhosting.com/support/website/ftp/getting-started-guide). Once connected to your server (you will need your FTP username and FTP password for this) you should see a number of folders in filezilla on the remote server.
You will need to navigate these folders to the broken theme and find the functions.php file. You will find it in: /public_html/wp-content/themes/twentytwelve/.
The functions.php will probably need to be downloaded to your computer and edited before re-uploading to the server again. You should be able to edit functions.php in a free text editor like http://notepad-plus-plus.org/ on your local machine.
You will need to scroll to line 500 and look for an error in the code. It will be something small like additional white space or perhaps an extra semi colon. You can always paste lines 495 - 510 back here if you cannot work out what needs changing.