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I see a scroll-bar around the webpage on the IFrame

If the dimension of the webpage you are trying to display within the IFrame exceeds the configured dimension of the IFrame it will automatically add scrollbars. Try to include a webpage that could fit within the IFrame.

I don't see any border around my IFrame

Borders have been deliberately turned off in the plugin. If you want to get rid of this you may edit the iframe-widget.php and remove all occurances of the HTML code frameborder="0".

How do I add an IFrame to a blog-post

To add IFrames to any post or page just add the markup [dciframe]url,width,height[/dciframe], for instance [dciframe]http://www.google.com,400,250[/dciframe]. Note that supplying the URL is mandatory while the width and height parameters are optional. Which means that you may specify only the URL or only the URL & width. Therefore, [dciframe]http://www.google.com,400[/dciframe] and [dciframe]http://www.google.com[/dciframe] are valid tags. Also note that the order of width & height is important and URL, Width and Height must be separated with commas. Lastly, the closing tag [/dciframe] is mandatory.

Can I add multiple IFrames on a Post or Page?

Yes you can. Just add multiple [dciframe] tags where required. All of these can be configured independently.

Can I add multiple IFrame Widgets on sidebar?

Unfortunately no. As of now you can only add one instance of IFrame Widget on sidebar.