I've used the following guide when changing the DNS settings on the MI424WR. I believe I got it from a post here some time ago; apologies for the non-attribution.
OK guys, I have just completed my considerably long and tiring experiments with the insane Verizon FiOS router, the Actiontec MI424WR. Some of the posts have indicated that there is something going on here, and since I think I know what I'm doing, I thought I'd take a poke at it, so I just spent my Sunday on this.
I cannot completely explain what I am observing---but I can tell you how to set up OpenDNS correctly, and at least pass along what I've seen with the behavior of this extremely complex and subtle (i.e., buggy) router. If you obeyed the instructions for this router on the OpenDNS site, you almost certainly THINK it's working, and it probably looks like it is. But it's not....not quite.
First, how to set up.
0) Go into your router (192.168.1.1). Select "MY NETWORK". On the left, select "NETWORK CONNECTIONS". You will now have a list of network connections the router manages, including your 'Network' (and its bridged sub-parts---Ethernet and/or coax and/or wireless), your two possible broadband entrypoints (Ethernet and Coax), and WAN PPP over Ethernet. If you got this far, you're going to make it. Trust me. 
1) Ignore the instructions you see under the OpenDNS mainpage for this router after this point. They are not completely correct. The instructions would have you change the DNS server entries under HOME/NETWORK. This is NOT the correct place, and will only PARTIALLY work (as described later in this post). Under HOME/NETWORK, choose "No DNS Server"---which is probably what it is set for already---unless you've tried the other instructions on the opendns site already and screwed it up! Return to the Network Connections page, and select either Broadband Connection (Ethernet) or Broadband Connection (Coax), whichever one shows "CONNECTED"---only one will be---on the right (Verizon has been mostly installing with the coax in Baltimore).
2) At the bottom, click SETTINGS to bring you to the "configure connection" screen.
3) Now, be very careful. If you do the wrong thing here, you will regret it.
Near the bottom, Find "DNS SERVER". Change the setting to "Use the Following DNS Server Addresses". Enter in the primary and secondary DNS Servers. Click Apply.
4) Now, your connection is almost certainly going to cut out for up to a minute. When it returns, any device that connects in any way to your FiOS will be DNS'd through OpenDNS.
And that's it. You're good to go. You probably won't even need to reboot. If you want to un-do the change, return to this broadband connection configuration and reset DNS server to "Obtain a DNS server address automatically".
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OK now to my observations. I observed a hell of a strange thing. If you change the DNS server in the Home/Network connection only (these are the main instructions on the opendns site), then I seem to observe the following:
1) Most new and valid DNS queries do in fact get serviced by openDNS.
2) Any test to see if you are using OpenDNS will generally pass.
Therefore, you probably think it's working... but:
3) When you attempt to block domains in openDNS, it is a crap shoot as to whether or not the blockage is active. This was the first thing that caught my eye that something strange was happening that had to be inside the router. For example,
I am a graduate of UNC. Therefore, my first blocked domain was, of course, duke.edu. After waiting the requisite time for that to take effect, I noticed that www.duke.edu still resolved, www.math.duke.edu was blocked, other depts were OK, others were blocked, it was crazy. (and yes, I cleared all local resolutions before doing this, etc., I'm not nubcake over here--it was definitely the DNS)
4) When you attempt to resolve an invalid address, you have about a 50-50 chance of hitting the OpenDNS page, or the Verizon spam/ad page.
So the astute reader knows that this surely indicates that SOMETIMES the router resolves with openDNS and sometimes with the deeper Verizon root dns. The only commonality I was able to deduce was that longish invalid names or names with a lot of numerics in them tended to resolve to Verizon's bad-dns-lookup page, whereas shorter/sweeter names tended more often to resolve to openDNS. I have no idea whatsoever what exactly is going on in the ActionTec, but whatever. It is completely insane, and defies my understanding, and I have a Ph.D. in mathematics, so I am well familiar with insanity, and I am still at a loss.
*IF* you set the DNS server in the deepest part of the router (i.e., right in the broadband connection itself, as I outlined above), then everything seems fine and 100% openDNS. If you set it only at the top level---which is the general instructions you originally get from opendns--- the home/network level, then USUALLY it uses those settings, and SOMETIMES it uses the Verizon default deeper settings. Whatever. Good luck, cousins.