You must be given alliance diplomacy role by your alliance to offer NAP to other alliances. With this role, on the Options tab of the Alliance page, you will have another option for alliance diplomacy and you can offer NAP there.
There is also such a thing as a personal NAP (Non-Agression Pact) This is just negotiated by in game message...and then if you wish be posted on your profile page using the "change the profile" option. Beware!! NAP's may be offered simply to give the offerer time to build up strength...this is a war game. So offering or accepting should be carefully thought out!