The "Trade Treaty" shows up under the Intelligence window in the same place as the Neutral/Friendly/Hostile relations options. Granting a trade treaty requires relations of at least 200.
However, Trade Treaties are a civilian shipping mechanic and will not move minerals. There is no way to trade minerals between different factions in normal play, but trade can be 'faked' by using SM mode if you control both factions.
To fake a transfer:
1) fly an empty freighter fleet of sufficient capacity from one location to the other and back. (optional: for RP purposes)
2) Go to your populations and use SM mode to increment/decrement the appropriate mineral amounts. I can't remember the exact button name, but it is one of the buttons that shows up at the bottom of the population/production window in SM mode.