From #openttdcoop wiki
Installing and running AutoTTD in combination with OpenTTDCoop is easy
First of all, download AutoTTD [here].. installing should be as straightforward as extracting the zip and running it.
Keep in mind that you set the proper directories when starting it first, if you messed up that part you can always change it from the menu.. both should be pointing to your openttd directory Also, if you do not run the default windows but a 64 bit windows, make sure to change the setting in the tools menu.
Now technically AutoTTD can already work for OpenTTDCoop by browsing the master server list, but to make things easier a few more steps to add it as a favourite (so you can easily refresh it)
Head to the Favourites tab, and add the following Favourite list url:
once that's done you can show all openttdcoop (and related servers) by selecting ' refresh favourites' from the arrow menu at the refresh button.. also the openttdcoop (and related) servers will show up bold when browsing the master server list.
You can join directly by selecting the server and hitting the Update/Play button .. if you want to show more details of the current game, click the More Info button... this will also allow you to join the company directly by selecting the company and hitting Update/Play on the server-specific tab that opened.