How to start alternate character missions in Arkham City?

I downloaded the Robin and Nightwing packs for Batman: Arkham City, but how do I start playing as those characters?How to start alternate character missions in Arkham City?
You can only use those characters in challenge maps. Catwoman is the only character with her own story.How to start alternate character missions in Arkham City?
You can only play as Robin and Nightwing in challenge mode. There is no option to play as them in the campaign.

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