I have created an IDF group and would like to create various AES design storms using the group. I am easily able to generate the 1 hr storm but I am not sure how to make the 12 hr Storm. I do not have the option to change the Mass Curve to the 12 hr curve

The AES is used to create a 1-hr AES design storm.
To have a 12-hr design storm, I can recommend you two solutions. First, you can apply “Create SCS Design Storms” or “Create Chicagos Design Storms”.
Second, you can calculate your “Mass Design Strom” and enter it into the resource library. To do so, you need to prepare your data in a specific format.