Hi Christian.
You need to use the Query String to compare the Start Date with the current date. I have created a test page and used the following Query String, and it worked. Please check and replace the same on the page you want.
post_type=mec-events&meta_query%5B0%5D%5Bkey%5D=mec_start_date&meta_query%5B0%5D%5Bvalue%5D={{dc:global:date format="Y-m-d"}}&meta_query%5B0%5D%5Bcompare%5D=%3E&meta_query%5B0%5D%5Btype%5D=DATE
Please remember that the above code will work if copied as it is and doesn’t conflict with any existing style.
Please note that the code provided serves only as a guide to help you get started custom coding on your own if there’s no option offered in our theme or the products we bundle.
We really do not provide support for custom codes, which means we can’t fix it in case it conflicts with something on your site, nor will we enhance it. Further customization should be directed to a third-party developer, or you can avail yourself of the One, where we answer questions beyond normal theme support.
Thanks