With MrSite, your domain name is tied to the plan you are on, and there is no ability to make CNAME changes (which is required to set up your store with SupaDupa). You'll need to transfer your domain name away from Mr Site, to a standard domain registrar where you can then configure your domain. This is a process independent to SupaDupa, but here's some notes for your convenience:

The process varies depending on your domain TLD (.com, .co.uk etc) and there is a £15 charge issued by MrSite.

The best place is to start with this article from the MrSite documentation. The process is well explained, please be sure to read and understand this: Transferring your Domain Name away from MrSite.

We recommend going with a popular domain registrar, such as GoDaddy.com. You can find notes for transferring a domain to GoDaddy.com here: Transferring a Domain Name to GoDaddy.

The process on GoDaddy starts here: Start transfer on GoDaddy.
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Once you have completed the move to GoDaddy, you can configure your domain with the assistance of our article here: Configure your GoDaddy domain name for SupaDupa.
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Needing further help? Contact support here at SupaDupa and we'll be happy to help!