Well Liz,
You are going to have to use a domain name thats NOT in use or use your ip to login to it, and you will have to set up your computer to do this, and it will only work until you clear the cache and then you will have to do it again. Much easier to use a new domain name, install osc to that, then REDIRECT the old one thru your domain server.
If you have installed it to an ADDON domain, you may be able to access it thru your main account domain name, but since i dont know HOW you installed or the basics of your account I cant say, but you could try
It would be
www.subdomain.maindomain.com/admin if the last was the case.
Karen