In order to forward your domain, you'll need to set up a CNAME redirect which sends the 'www' of your new domain to yourusername.clippings.me.
You can access these settings in the domains/DNS section of your domain registrar.
Once you find the DNS Manager, you’ll want to forward the CNAME record for ‘www' through to <yourusername>.clippings.me (this won't work as a regular web address). It can take up to 48 hours for this change to take effect.
After you've done that, you can log into clippings.me, click the cog at the top right to open account settings, and then click 'Domain'. Enter your domain name (i.e.www.yourname.com) in there. With the CNAME correctly added to your domain, the system should allow you to save the new domain and your new address will be working.