I really do think that advertising sites like cetrk.com, googlesyndication.com, adstronic.net, etc are wasting my bandwidth how can I block this site using OpenDNS's Content Filtering Categories? I do have a firewall and I still want OpenDNS as my fall back on this.

Thanks and I will always want to use OpenDNS.

You could also setup a transparent install of Untangle behind your firewall. There is ad blocking in the opensource package. It's a rather simple GUI install with decent content / protocol controls. There is a web based interface that noob could administer.

Pros:
One nice feature is it's temporary blocks; these are warnings about content that allow users to click through. It has the ability to block various file types and protocol. ClamAV can be setup to scan downloads. IP addressed sites can be block, one check box knocks out anything XXX.XXX.XXX.XXX, for those that aren't setup to pass.

Cons:
Content filter lists are rather slim, diffidently Not an OpenDNS replacement. Of course one could enter console to change that. High system over head, I would recommend P4 or better with a decent amount of RAM. It could not handle a pfSense failover (CARP).