To configure the reactions widget follow these steps.
1.Sign in to blogger account.
2.Go to layout Page.
3.Click on the "edit" link under the blog posts
4.On the pop up window check the "Reactions" widget.Click save.

If you are using the default blogspot template, the widget would appear in your blog.If not you have to add some code to your blog's template To do that follow these steps.
1. Go to edit Html page of your blog.
2.Check the "Expand widget templates"
3.Find the following code in your template:
<data:post.body/>
4.Then immediately after this line add the following code.
<span class='reaction-buttons'>
<b:if cond='data:top.showReactions'>
<table border='0' cellpadding='0' width='100%'><tr>
<td valign='center'><span class='reactions-label'>
<data:top.reactionsLabel/></span>
</td>
<td><iframe allowtransparency='true' class='reactions-iframe' expr:src='data:post.reactionsUrl' frameborder='0' name='reactions' scrolling='no'/></td>
</tr></table>
</b:if>
</span>
Save and Preview your blog.