thanks for the link and the description!
Now I now what you mean :-) The scrolling that you see in the preview is only possible because the table is part of an iframe modal window. Something like this is not really possible or useful when the table is shown on the frontend. However, there's different scrolling mechanism that you could you. You can see it in action here:
Now, unfortunately, that scrolling can not be used in WP-Table Reloaded, but in the official and designated successor of WP-Table Reloaded, which I'm currently working on. It is called TablePress and available at http://wordpress.org/extend/plugins/tablepress/ . However, to use this specific feature, you will have to wait for version 0.8-beta, which I will release next week, or use the current developer version from the GitHub repository.
So, my suggestion is that you switch now from WP-Table Reloaded to TablePress, and also already change the Shortcode of your table to
[table id=2 datatables_scrolly="500px" /]
Once you then update to 0.8-beta, the table will automatically use the scrolling.