First of all, are you clear about the difference between a sub domain and an add-on domain? If your domain is fred.com, then a sub domain would be support.fred.com, or charlie.fred.com.
On the other hand, an add-on domain on fred.com would normally not include fred.com in the URL at all - it would be just add-on.com.
It is possible you might want an add-on domain not a subdomain.
You say you have added your subdomain and installed WordPress but it doesn't seem to be hooked up. Well, when you install WordPress you get asked where exactly want to install it and you do this by selecting from a drop-down list. In that list will be any add-on domains or sub domains which you have previously set up. So it may be you have installed WordPress is selected the wrong subdomain/add-on domain when installing it.
In which case go and reinstall it, select the right place in the drop-down box, and you should be fine.
If this is the case then it's good practice to tidy up and go and uninstall the WordPress install which was an error. If you don't, then there will be a potential security hole at your host.
I hope this move you forward.