Well, I think I just figured this out as well, I had to download the source code version of the 5.1.4 and dig through the hostsettings, editschedule, and many of the Library files to finally figure out where it was looking for this information.  It appears that DNN decided to rename the web.config option to IsWebFarm, that solved the checkbox.  The second is that there is a table called WebServers that seems to auto populate some how (don't know don't care) and there is an Enabled column.  There is no interface in the community version to update this table, or even know it exists.  I have read through all of the documentation provided both by DNN and various "go to" blogs and found this no where.  I did see that there is an updated version of the PDF for web farm support in the Pro version of DNN, and also in the Pro version you get an interface to manage the web servers which I image affects this table.  It appears in the pre 5.x.x versions you just got a textbox for the scheduler items where you could type in your server names.  Now it is a checkbox list that is only populated by enabled entries in this table and the new mystery web.config option set to true.  Overall I have been extremely disappointed with DNN and web farm support/information.  I get the distinct impression they wanted you to buy the pro version to figure this out and I think that sucks.  Charging $2000 for a handful of modules seems outrageous, right up there with paying Geek Squad to install RAM in a PC.