Sorry if this is already posted, but I found a way to get the mc:// URL without the server issuing the URL for you.

First, go to the regular Minecraft page of the server* you want to use World of Minecraft in. Then, view the page source. (If you're using Firefox, it's View->Page Source, or Ctrl+U) You're going to need to find a block of code that looks like:
<param name="username" value="SlyCooperMan">
<param name="sessionid" value="-875352396951892878">
<param name="haspaid" value="true">

<param name="server" value="61.19.248.94">

<param name="port" value="32701">
<param name="mppass" value="583352552d6ac6bccb83f584aaa7fc3a">

(Sorry, I couldn't get <code> to work)
This should be right above the "Controls" text.

Now, get the game client opened up. In the Direct Connect URL box, type in mc:// followed by the IP address given in the "server" value code, in this case it will look like mc://61.19.248.94, Then type a colon followed by the numbers given in the "port" value code then type a slash; it should look like this: mc://61.19.248.94:32701/. Now, type your username you are logged in as then another slash; "mc://61.19.248.94:32701/SlyCooperMan/". Finally, find the code given by the "mppass" value and type that in;"mc://61.19.248.94:32701/SlyCooperMan/583352552d6ac6bccb83f584aaa7fc3a". After that, you can hit "Go!" and you should be connected.

Thanks.

EDIT: If during the middle of your game you lose connection, it's just because the mppass has changed. First, use the same URL again, and if you get a log in error, just go back to the normal server page, view the source, and copy and paste the code again. All the other parts won't need to be changed.

*So if you don't know what I mean by that, I guess I'm going to have to go back to the basics. Log in, go to the main page, there should be a link to the right that says "Multiplayer" under Classic, that will lead you to the classic server list, find your server, click on the link, and then do the page source stuff from there. Mppass does exist.