If you have an existing domain name, you will also need to point the domain to your new SiveHost web hosting account. You can either change your name servers at your existing registrar, or transfer the domain to SiveHost.
If you are not sure which to chose, you may want to google:
Change Name Servers for your Domain Name
If you want to keep your domain registered with its current registrar, then you would change the name servers to point to SiveHost. The following article gives details on how to do that.
Transfer Your Domain Name Registration to SiveHost
You can also transfer your domain registration to SiveHost so you can manage your website and domain name with one company. The following articles will assist you with that.