Well, don't know really why your not getting that button in game. Perhaps try a few different start options.
Just tried it again like this: (its from german version)
and then i get the button in game immediately at the start of the game:
I also tried with 6 AI players all @ team X and also got the button in game.
The only time i don't get that button to access the diplomacy options is when all players are in one team vs me from the start.
Also note, that after you have for example set up an alliance for 2 minutes and paid for it, the button will dissapear untill the alliance time is past. Then it will give you a message that the union is over an the button will appear again.
Not sure if this helps, as I said, try out a few different start options, I cannot try them all out as there are too many. However, if you would like to read additional information on the various new options from Imperia , pls see here : http://www.internetwars.ru/Imperia/ImperiaEnglish.htm
Let me know if you got it working, because its a rather cool aspect to the mod.