when using advesary dice, do you use it in an opposed force attack with aswell? say attacking a Sith lord, or the sith lord using force power on the players in a contested roll

I would say Yes.

when using advesary dice, do you use it in an opposed force attack with aswell? say attacking a Sith lord,

Yes, although I don't add Adversary dice to non-combat. The reason is combat has a fixed difficulty independent of the target, so Adversary upgrades make sense as a way to make the target more dangerous to attack. But the target will likely already have decent Discipline/Willpower/Cool, etc, so adding the Adversary dice to those pools seems like overkill.

...or the sith lord using force power on the players in a contested roll

If the Sith Lord is using a Force power on the players, they are using their own positive dice against the player's negative dice, so you don't add their Adversary ranks to their positive dice.

Generally speaking, I wouldn't, as the Adversary talent is meant to be used against your standard combat checks, such as Brawl, Melee, Gunnery, Lightsaber, and Ranged (Heavy/Light).

In terms of an opposed check to resist a Force power, in most instances the Force user isn't necessarily making a "combat check." Don't have my book handy, but I believe that Move and the Unleash side of Protect/Unleash specifically cite the need for a combat check, with Move being to hit somebody with a hurled object and Unleash using Discipline to hit someone.

In terms of resisting a Force power, most major Force users are going to have a decent Discipline pool (at least three dice, the majority of which are yellow), so allowing Adversary ranks is probably overkill unless your PCs have very heavily focused on raising their Willpower and Discipline ranks.

Where a PC is making a check which is an "Opposed Check" or "Competitive Check" to do something to an NPC (talk, force power, hide from, race against ETC) then I don't include the Adversary talent, as the NPC's skill ranks will already be upgrading the check.

Wher a PC is making a "Basic" skill check that directly affects an NPC (mostly a combat check) then the Adversary talent comes into play.

Yeah, just keep it RAW (Adversary applies to combat checks) and give your NPC ranks in whatever skill you think he should have.

Thanks all!

I'm going to agree with the above and adjust my answer to Yes for combat and not so much on opposed checks.