In AdEspresso, it is not possible to add a new ad into an adset. If you are looking to insert a new ad, you have two alternative ways of continuing:
1.) Edit a pre-existing creative and update it with the graphics/creatives you want to add. You can edit your creatives and targeting from the All Ads page. Just click from the campaigns dashboard to the all ads page -> click show ads -> edit creatives. Instructions on how to edit your creatives can be found here.
2.) Copy your original campaign and publish with new creative. Pause the campaign, copy it, add in your new creatives, and publish this new campaign. You can find instructions on how to copy your campaigns here.