Although I'd never heard of MrSite before, I took a look and it seems pretty clear to me (others can feel free to correct me) that you cannot install WordPress software on MrSite.
Although they combine the two, registering a domain name (yourname.co.uk or whatever) and hosting that domain name are two different things. If you look around, you can generally find domain name registration for about $10 U.S. a year. My point is that MrSite will cost you many times that price if you use them just for domain name registration (because you can't use their hosting for WordPress).
If you are not sure that WordPress is where you want to go until you've tried it, why not try it out at wordpress.com for free? No installation required.
Asking MrSite for a refund would normally be my first recommendation, but it can be tough to get a refund when a domain name registration is involved because MrSite cannot cancel a domain name registration and get their money back.