If you'd like to add a Facebook Like button to your posts, just copy the script below and follow the instructions for pasting that into your templates.
<!-- facebook like -->
<iframe src="http://www.facebook.com/widgets/like.php?href=<$MTEntryPermalink encode_url="1"$>" scrolling="no" frameborder="0" style="border:none; width:450px; height:80px"></iframe>
Add the code in the Individual Archives template and within the MTEntries container tag in the Main Index and Archive templates. The button can be placed wherever you wish but it's generally added in the post footer area (MTEntriesFooter).
You can adjust the above width to fit into your sidebar. For more customization options, check out Facebook's Social Plugins pages.