Cascading Proxy Server

I am running Backoffice Small Business Server 4.5 on a Dell box. My ISP is Integrity On Line. When I use Proxy Server, the clients can only browse my ISP’s home pages. The usual setup for a stand alone computer is to put “proxy.iol32.com” as the name of the proxy server in IE5 properties. To use Microsoft Proxy Server, I have to put the name of my local proxy server there instead. Where do I put my ISP’s proxy server name?