Hi, I'm reading the rules of the campaign and I see that the imperial can acquire agenda cards as usual, but in the campaign there is no mission stage, so, how can the imperial gain villains if he can't force the players to choose a mission? The imperial wins no villains as usual in tyrants of lothal? Or, at the moment he puts in play the agenda card gains automatically that villain? And, what happens with the forced missions?

For mini campaigns, all agenda cards that put missions into play are removed from the deck.

See above (and the mini campaign setup rules).

The imperial player can earn villains during minicampaigns using other methods like abilities on the Nemeses and Overwhelming Oppression class decks.

Note that in a minicampaign all missions are considered story missions, so some agenda cards that give single-use villains do not work (by rules as written).