I don't see a parked domain for your account at this point.
What are you trying to set your main domain to be? Do you get any errors when you try, if so please include the error message as that helps us sort the issue.
I have used the x10hosting control panel to change my main domain to wallcity.tk. But it seems no effect when I surf my domain wallcity.tk, it shows a wrong page. But when I surf the domain sshfree.tk that I changed before, it works.
The default page usually comes up for a bit until the system is fully updated with the domain change. I'll kick off a little command on the server to speed it up a little. Give it 15 minutes, clear browser cache and try again.