The way it is set up, you have to use one of the x10hosting's options for your main domain name.
You can then use your purchased .com name as either
1) Parked domain -- becomes an alias for your main domain. Exact same content. Visitors to mywonderfulsite.com never know that it is an alias for areyoukiddingme.x10host.com
2) Add on domain -- a separate site from your main domain. Different content. Stored in a subdirectory of public_html