multiple columns in content block
if its not already on, you should be able to get to a toolbar that lets you build tables by changing the tinyMCE settings in the dashboard...
click "more toolbars" up top in the content block.
[Edit: The tinyMCE table editor works in Firefox.]
Oh, and put your table in <div></div> tags.
No doubt this was already answered for you but perhaps can help the next person searching for an answer.
Why not create some floating classes and clearings in your main.css and use these when adding content?
check out theme column cruiser, it's based on yaml and show how to use liquid div boxes for min-max width sites, that fit various resolutions.
<div style="column-count:2; -moz-column-count:2; -webkit-column-count:2;">
YOUR CONTENT HERE
Today a better solution is to use floats for columns so they can work on both laptops and phones. But not sure how this would work with concrete5.