try to go to your "root" - "templates" - "rt_entropy_j15" - "css" and there you have the extensions.css file. Open with an editor and search for ".roktabs-wrapper .roktabs-container-wrapper".
This have the strings "clear", "position", "overflow" and "width". Try to add:
margin-top: 25px;
...or -25px or:
padding: 25px;
or -25 ... i think, you understand, what i mean.
Padding defined the internal space, margin the outer. If there are no changes, or not that changes, like you wanted, make the changes undo and try another div-container. You can search for it with the Firebug for the Firefox Browser, or the standard build ins in Chrome and Opera. Just a right click on the position, you want know more about - something like "analyze elements".
The problem is that you are running Jquery.
There is a Mooscript that calculates the right distance to the first section when the fixed-header is active. (it injects an empty div to push the content down) That script fails when you have the Jquery stuff loading. So try to get rid of it.
I'm pretty sure the mailchimp script is responsible for this. Try to disable that first.
You can also try to enable the Mootools upgrade plugin ...