It couldn't be simpler: just sign up for a free forum at Forumotion and you can add it to your Jimdo site! If you've been looking for a way to do this, just head over to Forumotion now and you can get started.
Step 1: Insert a new Widget / HTML element.
Step 2: Copy and paste the code below, replacing the URL with the url for your forum
<iframe width="800" height="1000" frameborder="0" marginheight="0"
Note: Don't forget to replace xxx.forumotion.com with your forum's address!
Unfortunately, it's not possible to change the size of the forum. You do have the following options...
Where it currently says width="800" height="1000" as the dimensions, you could reduce it to width="500" height="700". This makes the forum smaller (and maybe it'll fit better on the page) but your visitors may have to scroll horizontally and/or vertically in order to see everything. It's not perfect, but in short, your site does gain some interactivity. ;-)