Once you have your My Docs Online account set up it is fairly simple to configure OmniFocus to work with our WebDAV servers. On the desktop side just launch OmniFocus and from the menu bar click OmniFocus, Preferences. From the preference window click the sync button at the top. Then select the advanced button and enter the following location address: https://webfolders.mydocsonline.com. Please refer to the graphic below for an example.

Once you have the address entered click the sync button and you'll be asked for your My Docs Online login ID and password, following which the synchronization will be initiated.
Step two is to configure the OmniFocus app on your iPhone. You'll first need to purchase and install the app from the iTunes store. Once the application has been installed go ahead and launch it and then tap the "Settings" icon. On the ensuing Settings page scroll down to the Syncronization section and enter the same URL, https://webfolders.mydocsonline.com. Please see the image below as an example.
Once again you'll need to enter your My Docs Online login ID and password here after you've entered the WebDAV server address.
Please review the instructions from OmniFocus for complete details on how the synchronization works. For the time being the only version of OmniFocus that supports the iPhone syncronization is version 1.1, which they refer to as a "sneaky peak" version.