This is an old post and Issuu has made it super easy to embed Issuu into posts. Now you can simply click share on the page of the issuu file you want to share, then make sure you click on “Use on Tumblr, WordPress, etc.” and they'll give you an iframe embed code you can simply past into your post like I did above. You no longer have to use special plugins to achieve this.
Make sure when you post the code, that you post it in while in the “text” window and not the “visual” window in while you're creating your WordPress post. Otherwise it might not work. If you want to resize your Issuu embed then you can switch back to the “visual” editor and just resize the box by dragging out the bottom right corner of the yellow box.