MySQL databases have a table for users, which contains privileges etc. It appears your user doesn't have that FILE privilege - if you don't have access to that table, emailing the host may be your only option.
Before you try that, try using the LOCAL command:
mysql_query("LOAD DATA LOCAL INFILE '/home/username/public_html/Database.txt' INTO TABLE yourtablename") or die("MySQL - Query Error - " . MySQL_Error());
The local command means that the file is retrieved by the client - e.g. PHP - rather than the database server. I'm not sure whether this will have an effect, but it's a possibility.
If all else fails, you could perhaps load the data into a local mysql database, then export the table (with IF NOT EXISTS on the creation), compressed, through PHPMyAdmin. You can then import said table with PHPMyAdmin on your server.