If you want it to refresh the CMS home page you need to edit the template vBulletin CMS Templates / vbcms_page
If you want it to refresh the Forum home page you need to edit the template Forum Home Templates / FORUMHOME
Add the following code to the very top of the template.
Code:
<meta http-equiv=refresh content="300">
You can change 300 to what ever you wish, the value is in seconds.