First, what's not working? No redirect? No 301 redirect?
Second, in Site Manager>Settings>URL & SEO>SEO, make sure you have "Allow permanent (301) redirection" selected. Also ensure you have "Redirect document aliases to main URL" selected.
Third, pay attention to where you're placing the URL alias or redirect. If you've selected the 2 checkboxes above the the following should work fine. Say you have a page /About. On your old site the URL was /AboutUs/Default.aspx. You want to navigate to the page you want your viewers to end up on (in this case /About) and add an alias. Your path would be /AboutUs/Default and your extension would be .aspx. So now when the viewer uses their bookmarked favorite and navigate to /AboutUs/Default.aspx, it will 301 redirect them to /About.