If the widget is not showing up on your blog, try the following:
Maybe the post feed is blocked in your Blogger settings. Go to your Blogger dashboard and click "Settings" in the menu on the left. Scroll down - to the "Site feed" section, and click "Allow blog feed". Set it to "Full", click "Save" and check if the widget works properly now.
If this does not help and if you are using the widget to display posts from another blog, double-check that you have correctly entered the address of the blog from which you want to display most commented posts. Remember to enter the address to the blog's home page, and not to any subpages (even if the widget is to be displayed on a subpage).
Otherwise, check your internet connection.
If the widget displays on your site but does not respect your settings, try the following:
Do not use the translation of this page by the browser when setting and installing the widget, because if some settings in the generated code are translated by the browser, the widget will not understand them.