Hi @matthewskeith
The code in your functions.php file will need to be modified to look for the $_GET value within the serialised array string.
If you look at the docs for the checkbox field, you will notice that one of the code examples hows how to filter posts via a checked value:
http://www.advancedcustomfields.com/resources/field-types/checkbox/
Changing your meta_query to use a LIKE compare as shown in the code example will correct your code.
Thanks
E
