First, of course, make sure you're editing (or adding) a Post or Page
In the visual editor, locate these buttons:
To insert a single listing, click that first , click the first balloon icon
This should bring up a prompt, which will ask you to insert an MLS#. Enter the #, and review the rest of the information (although you'll likely leave everything checked as is) and hit Insert Listing when you're ready.
This will enter a shortcode into your Visual Editor. Make sure to enter some unique content above that shortcode (i.e. a unique description of the listing, or circumstances under which you're sharing the listing, i.e. open house, price reduction )
If you want to see what this translates to before posting, make sure to click the Preview button on the right in the 'Publish' Module
When you're ready, just click publish or update!