There is already a Like button on Facebook pages if the user viewing the page no longer looks like a page. If you want to add a similar button on your page for another object (for example, a website, article or another page), you will need to set a tab on your page where you can place the HTML content.
There are several third-party applications that allow you to do this, one of the most popular - http://www.facebook.com/apps/application.php?id=190322544333196 - you can add a new tab to your page with some kind of static content ( images, etc.) and enable the βLikeβ button on this tab.
It is not possible to embed a similar button for an external object in a message on the page wall that I suspect you are trying to do.
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