In your Jimdo page editor,

1. Add an element. (A plus icon situated appropriately at the left).

2. Using the Widget/HTML element chosen in step 1, input your form's iframe code.

Don't know how to get your iFrame code?

That's easy. 

In your JotForm editor, go select  Setup and Embed (at the top toolbar) > Embed > iFrame.

You'll have a code similar to the screenshot below:

3. Save then preview your page. It should show your pretty form made with JotForm embedded on a Jimdo page.