The Comments widget is a part of the CommentAce plugin so please make sure that it’s active.
The widget can be located under the WP Dashboard > Appearance > Widgets page by the (CommentAce) Recent Comments name.
|Number of comments to show||How many comments should be loaded|
– Most voted
|On a single post, show only comments from that post||Narrow comments to the current post|
The CommentAce Comments widget allows you to show also animated GIFs, not only text comments.