There are 2 steps to hosting a domain. 1) Set the nameservers which it sounds like you've done, and 2) Add the domain to cpanel, which it sounds like you haven't done.
If you want the .tk domain to show the same as your .heliohost.org domain then use the alias button in cpanel, and if you want the .tk domain to be something separate from your main domain use the addon button.
Depending on whether you use alias or addon it means both or either.