First you need to set a CSS class to the login form. To do so click Edit link for Login widget and write a CSS class in CssClass field like show on the image I have attached.
After that add a CSS widget ("Scripts and Styles" section in the toolbox) to the page and write the CSS rule from below in the text area in "Write CSS" tab.
margin: 0 auto;
Let us know if you need further help.
the Telerik team