First of all, check that you don't have locked teams enabled.  Locked teams prevents you from making (or breaking) alliances.  Great for multiplayer, probably not what you're looking for in singleplayer.  Second, you can't make alliances or pacts if this is a 1v1 game.  You can only make alliances if there are multiple AI's.

Third, you must build a relationship with the AI.  That involves completing its missions and sending envoys to its worlds to boost your standings.  Once they're high enough, the AI will propose treaties, and you can also form pacts if you have them researched.

You can only request to form a pact if it is one that you have researched. That means if the two of you are different factions, the pacts from the other player are only available if they offer it to you.

Pacts run both ways. Pacts can be offered and pacts can be received.

Pacts have certain requirements that must be met before they can be offered though.

The first requirement is Envoy. If you want to give pacts send one envoy to each planet of the ally you want to form the pact with. This is also true for your allies in that they will need to send Envoys to your planet before a pact can be offered to you. The role of the Envoy is to build relationship points between you and an ally. Opening the Relationship window will show your relationship with the other players. You can also see the other players relationship scores in that screen as well by clicking on them.

Once the relationship hits 11.5 then you can offer a pact to your ally. Again the relationship point is a one way affair. The AI or other Ally will have to build it's own relationship with Envoys to 11.5 (or higher for better pacts) with you to offer their own pacts.

The next requirement is to research pact types. Only pacts that have been researched can be offered.

The receiving side of the pact does not have to do anything but agree to accept. In otherwords you or your allies can accept pacts from other players without ever building a Envoy or researching pacts.

Probably the easiest way to improve your relationship is to attack a third player (that is hostile to the player you're trying to form a pact with). This will start boosting your Military Actions with that AI. Although you can get 20 relationship from that, only up to 10 will count to the overall relationship (the other 10, however, can be used to cancel any negatives you have).

Well, relationship gained by resources given will be slowly lost over time, and often you want to research only what is necessary.