Welcome to the forums! Lets get to the bottom of your question.
Please follow these steps to cast a spell on an incoming/outgoing attack.
First click the icon that the arrow points to. The icon may look different depending on if you are attacking or you are being attacked.
Once you click on that square icon a dialog will open up. Click on spells on the upper right hand corner and a list of spells will become available for that attack. I have a screenshot but the only god I currently have on that server is Hera, if you have a different god or more than one god those options will open up for you as well.
Once you activate a spell you will get a dialog that states your spell was successful, however if the other player sending/receiving the attack casts a divine power before you do, you will get an error message stating a divine power was already enacted on this attack.
When you send support to another city and it contains no mythical units it shouldn't matter what god you are worshiping in your city. Make sure you are not sending myth units when you attempt to send support. If the problem still persists try a few steps. First clear the cache on your computer. This will solve most problems. Info on how to do that can be found here:
How to clear your browser cache.
If that doesn't work try to use a different browser or computer. If you still have no luck please send us a support ticket
here. Make sure to include the issue you are having, and which steps you tried. We will look at the account and get back to you as soon as possible.