How can I add a scroll to my website to accommodate my long dropdown menu?

Hi guys! 

My website is below:

producedbybeckert.com (password is "BRANDED")

You'll notice that if you click on "Conversations," and you're on a standard macbook laptop- you can't even scroll down to see all of the links that appear under the "Conversations" dropdown link. 

my question is: how do I fix this? how do I add the ability to keep scrolling? Right now my website is fixed on my page, so i can't scroll down to the bottom of the dropdown menu. 

Add to Design > Custom CSS

.main-nav .subnav ul {
    max-height: 500px;
    overflow-y: scroll;
}