Regarding the Custom Field Type, I suggest that you use the Advanced Custom Fields Pro plugin which is already available as a bundled extension in our theme.
Using that plugin you can easily add a custom field to any Custom Post type you want including the event calendar and you do not need to delve into the code so much.
I am not sure regarding the MEC Category link issue that you mentioned. I wonder if you are using the latest version of the MEC available? You can test that by going to plugins menu and see if there is any message regarding the update for the event plugin, if yes kindly update it.
If you still experiencing the issue please kindly get back to us with detailed information on how you added the category to the sidebar and login information in a Secure Note to follow up.