You need to basically tell your domain name to "point" to the Dotable servers. This is done by changing the DNS information for that domain name.

You will need to access the domain register that you purchased your domain name from and there should be a member's section. This will require a Username and Password [that you received when you bought the domain name]

Then find the section that says something like "Change or Update DNS info". You then replace the information that is currently there with the nameserver information that came with your welcome email.

By entering this nameserver information, you are instructing that requests for your domain name are directed to the Dotable.com servers. It could take up to 3 days for your domain to be fully propagated and for your site to be accessible via your domain name.

If you can't find a Member's section on the site that you purchased the domain name from, email their support staff and tell them what you want to do etc.

By redelegating your DNS info for your domain name, you are in effect hooking your domain name up with the Dotable.com servers and any requests for your domain name will be routed to the Dotable.com servers.

Your nameservers to use are in your Dotable.com welcome email. If you need more information, open a ticket on the helpdesk and we'll provide fast assistance.

Here are some live demos showing you how to change your DNS at some popular domain registers -

Changing DNS at GoDaddy.com
Changing DNS at MelbourneIT.com.au
Changing DNS at NameCheap.com
Changing DNS at NetworkSolutions.com
Changing DNS at OpenSRS.net
Changing DNS at 000Domains.com
Changing DNS at Register.com
Changing DNS at Dotster.com
Changing DNS at StarGate.com
Changing DNS at Enom.com
Changing DNS at DRAMS