Well, something that took me a little bit of time and I want to share. I download the login widget and I decompress it. that give me the folder widgetlogin that I put on the root of the silverstripe instalation. Everything was ok, the widget works over the Blog module, the only thing was that the input texts where way bigger than the widget itself, actually they where as big as the input text on /admin login page.
I went to the CSS of the widgetlogin and I found that the input.text was well defined... but it wasn't affecting the widget in site. So I found that inside of LoginWidget.php, the line that include the LoginWidget.css does it from the directoty "widgets_login/"...
so if someone goes thru the same issue, the solution is:
to have the Login Widget in the widgets_login folder...
or to modify the loginwidget.php changing the line:
(change customfolder for the folder you actually have with the Login Widget.
and that's it!