It is possible to create a new campaign based on an existing campaign, directly from within Act!. To create a new campaign, launch the add-on, select your list in the main window then go to Campaigns>New Campaign.... This will launch the New Campaign Wizard.
First you need to give a name to your campaign (1) and select the existing campaign the new campaign will be based upon (2). This is the campaign the plugin will replicate to create the new campaign.
The existing campaign drop-down list contains all of your past campaigns and all your unsent but ready to be sent campaigns. In other words, all the campaigns that can be duplicated and sent/scheduled right away. Any campaign which setup is incomplete (for instance a campaign that you saved before completing the online wizard) will not appear in the list.
In the next screen, you need to select your campaign target.
NOTE: In previous versions of the Mailchimp Plugin, you could export contacts while creating a campaign. This is not possible anymore. So you'll need to make sure the Mailchimp group, tag or segment which is going to be the target of your campaign contains all the proper subscribers before sending the campaign.
The rest of the wizard should be self-explanatory.