To use iframe function you need to:
1. Create a link (a href="”#”").
2. Add the rel attribute “prettyPhoto” to it (rel=”prettyPhoto”).
3. Change the href of your link so it points to the webpage you want to open.
4. Then add “?iframe=true” as a parameters in your HREF so prettyPhoto knows to open the content in an iframe.
5. Then add the width and height as parameters in your HREF (&width=100&height=100). Please note that the dimensions can be percent based.
code example:
<a href="http://www.google.com?iframe=true&width=100%&height=100%" rel="prettyPhoto[iframes]" title="Google.com opened at 100%">Google.com</a>
<a href="http://www.apple.com?iframe=true&width=500&height=250" rel="prettyPhoto[iframes]">Apple.com</a>
<a href="http://www.twitter.com?iframe=true&width=400&height=200" rel="prettyPhoto[iframes]">Twitter.com</a>Have a look at this page for more informations: http://www.no-margin-for-errors.com/projects/prettyphoto-jquery-lightbox-clone/
Create a new thread and tell me what problems you are encountering and what you want to know. 
There is no step 3 :) 
