You need to go to your registrar and change DNS settings for your domain. You create a new CNAME record for 'www' alias with 'wackwall.com' (or 'xitti.com') value.
Attention: If you also want top-level domain (not only 'www.mysite.com' but also 'mysite.com') work with this feature you also need to set up A record. For WackWall the IP should be 173.203.196.228 (for Xitti: 204.232.202.47).
This is basically what you need to do. If it's not obvious how to do it with your domain registrar you may want to ask for their assistance. For example, in GoDaddy you need to go to TotalDNS settings to create a new CNAME record.
If you already use your domain with a hosting account and want to use its subdomain for a social network from WackWall you need to add corresponding CNAME record on your hosting account accordingly. Check your hosting provider documentation.
After creating CNAME record for your domain you need to go to your WackWall dashboard (or Xitti dashboard). Click settings icon on your social network and enter your custom domain.