To access your MemberKit website via a custom domain (eg. mydomain.com) you'll first need to make sure you have the appropriate DNS records for your domain pointing towards the correct MemberKit server (the IP address will be provided in your initial setup email).
In your domain DNS records, you will need to add or modify the primary A record to the IP address provided. As an example, in GoDaddy the entry will look similar to this:
Where @ is used by GoDaddy to indicate the primary A record (you may have other A records corresponding to subdomains) and 1.2.3.4 will be replaced by the provided IP address.
A temporary URL will also be assigned which you can use to set up your memberships, website content, templates and so forth before 'going live' to the public.