Please download and extract the zip file so that you have an xml file.
Login to the admin side of your store and go to "Manage Hooks" and click on the "Import Code Snippets" tab. Select the xml file on your computer and click import.
After import you can change the threshold level by clicking on "Disable checkout at total threshold" in the code snippet tab. In the text field you will see the PHP code. The value for $max_order_value can be changed to the level you want. Please also note that this is hard coded to £ GBP with £. This can be changed to any other curency symbol such as $.