The code Issuu provides isn’t directly allowed in wp.com blogs, for security reasons. But Issuu docs are flash, so they can be inserted via the gigya shortcode (see here for details). The shortcode can be pasted into the HTML post or page editor, or in a text widget for your sidebar.
Click the “Get Embed Code” button on the Issuu page, [Update: then click for "the old embed page"], change options if you wish or if necessary (check my posts on maximum main column width and sidebar width). Copy the Embed code and paste it into a simple text edit file. The code will look like this:
Copy the sections I have highlighted in bright red and turn them into a working shortcode this way (dimensions can be changed):