If you want to add external css on visual webpart,
- First go to layouts folder in 14 hive, path is C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS
- Create a new folder in it and give a proper name to that folder.
- Add your css files in that newly created folder.
- Now to call the css file from your webpart code, add below tag in your ascx file <SharePoint:CssRegistration ID="cssReg" runat="server" Name="/_layouts/ForCss/style.css"></SharePoint:CssRegistration>